IMO B,

We are asked which hypothesis explains the phenomenon in the argument. So we need to assume each option as a hypothesis that is true and evaluate it to see whether it serves as an complete explanation to what is given in the argument.

A: The presence of unrelated juvenile and adult male monkeys causes more self-destructive behavior in juvenile monkeys than does the presence of a mixed-gender group of unrelated adult monkeys.
This option suggests the presence of juvenile and adult monkeys causes more self-destructive behavior. However, it does not explain why the monkey is calmer when surrounded by a mixed-gender group of adult monkeys. Hence out

B: The presence of unrelated adult female monkeys is more important to juvenile monkeys in alleviating distress caused by being separated from their mothers than is the presence of unrelated juvenile monkeys.
BINGO! This one clearly explains the entire phenomenon by stating that presence of unrelated adult female monkeys contributes in alleviating distress caused in juvenile monkeys by the separation from their mother. This explains the juvenile monkeys behavior in each of the case. Hence correct

C: The presence of unrelated adult male monkeys is threatening to juvenile monkeys, causing self-destructive behavior similar to that caused by the separation of juvenile monkeys from their mothers.
Again this explains only a part of the phenomenon as to why the juveniles are more self-destructive in a surrounding accompanied by male adults. It does not explain other side of the phenomenon. Hence out.

D: The presence of unrelated adult female monkeys serves as a suitable substitute for a juvenile monkey’s mother following a separation from the mother.
This one is close. However, it is wrong. Mark that there is a little self-destructive behavior from the juvenile monkeys still, suggesting that adult female monkeys some what help in reducing distress but cannot completely substitute their mother. If Option D hypothesis was to be true then adult female monkeys would have been a good substitute and there wouldn't have been any self-destructive activities present. Hence out

E: Juvenile monkeys that display self-destructive behavior will engage in this behavior the least when reunited with their mothers, and will engage in this behavior slightly more when caged with adult female monkeys who are not their mothers.
This option is a little out of scope as it does not contribute in explaining the phenomenon given the argument. Hence Out

Hence B is correct

Hope it is clear

IMO B.

A: The presence of unrelated juvenile and adult male monkeys causes more self-destructive behavior in juvenile monkeys than does the presence of a mixed-gender group of unrelated adult monkeys.This option says the presence causes self-destructive behavior but it is the separation from the mother that causes this behavior.
B: The presence of unrelated adult female monkeys is more important to juvenile monkeys in alleviating distress caused by being separated from their mothers than is the presence of unrelated juvenile monkeys.This option rightly says that the presence of other adult female monkeys helps in alleviating the distress caused by separation from mother.
C: The presence of unrelated adult male monkeys is threatening to juvenile monkeys, causing self-destructive behavior similar to that caused by the separation of juvenile monkeys from their mothers.Nothing in the passage suggests that the presence of adult male monkeys is threatening to juvenile monkeys.
D: The presence of unrelated adult female monkeys serves as a suitable substitute for a juvenile monkey’s mother following a separation from the mother.The presence is not a substitute because self-destructive behavior is not completely absent but it is at its least when adult female monkeys are present.
F: Juvenile monkeys that display self-destructive behavior will engage in this behavior the least when reunited with their mothers, and will engage in this behavior slightly more when caged with adult female monkeys who are not their mothers. This option is too definitive. It says that the juvenile monkeys WILL display self-destructive behavior even after reuniting with their mothers.

I guess between A and B,
A only talks about one reason why juvenile monkeys show less self-destructive behavior
But B is more relevant to the conclusion because it explains the "still observable" part of the conclusion, which generalizes the WHOLE arguments and says that: The presence of mixed gender group unrelated adult female monkeys is more important to juvenile monkeys in alleviating distress caused by being separated from their mothers than is the presence of unrelated juvenile monkeys. (less stress therefore least prevalent)
The trick is not to stop at one justification, but look at the conclusion as a whole

OFFICIAL EXPLANATION

The question stem presents a series of premises about how juvenile monkeys behave when separated from their mothers; specifically, the behaviors are more or less prevalent under various conditions. We are asked for the hypothesis that would best explain the variations in behavior. Although we are not asked to draw a conclusion from the premises, we should look for an explanatory hypothesis using the same constraints: assume as little as possible and take no unnecessary logical leaps.

(A) This choice confuses alleviation of self-destructive behavior with causation of that behavior. The question stem attributes the behavior to separation from the mother, not to the presence of other monkeys.

(B) Correct. We are told that separation from the mother causes self-destructive behavior that is lessened some in the presence of other, unrelated juveniles and adult males, and lessened even more in the presence of a mixed group of unrelated, adult monkeys. Since adult male monkeys are present in both cases, they may be disregarded as a cause of the difference in degree of self-destructive behavior. The best hypothesis is that the distressed juveniles are more comforted by the adult females than by the other juveniles.

(C) We have no reason to believe that any other monkeys are “threatening”—merely that some monkeys alleviate distress better than others.

(D) This choice is on the right track, but its language is too strong. We know that the presence of unrelated adult female monkeys is more soothing than the presence of other monkeys to a juvenile that has been separated from its mother, but we don’t know if the juveniles are comforted to such a degree that the unrelated females are a “suitable substitute.” (For instance, perhaps the presence of other juveniles alleviates the behavior by 5%, and the presence of the unrelated females by 10%).

(E) The situation described does not mention reuniting monkeys with their mothers. Stay away from choices that require making additional, unnecessary assumptions.
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I think this question is problematic.

To explain the hypothesis the explanation should be both inferable from the passage and must be true in certain condition.
But, in the passage, we can only infer that either mixed gender or age of monkey (at least) can affect monkey`s behavior.
We can not conclude which one(or both)really affect the behavior.

B seems not crystal clear answer for me

1) The behavior is the worst when monkeys are caged alone.
2) It still exists but is not quite as bad when monkeys are caged with other unrelated juvenile monkeys and adult males.
3) It still exists but is even less prevalent when monkeys are caged with other unrelated adult male and female monkeys. ("These behaviors are . . . the least prevalent, but still observable, when the monkeys are caged with a mixed-gender group of unrelated, adult monkeys")

So which answer choice best explains the result? Let's compare:
B-The presence of unrelated adult female monkeys is more important to juvenile monkeys in alleviating distress caused by being separated from their mothers than is the presence of unrelated juvenile monkeys.
YES. Compare 2 to 3. The monkeys had fewer signs of distress when with the adult male and females than with the juvenile and male monkeys.

D-The presence of unrelated adult female monkeys serves as a suitable substitute for a juvenile monkey’s mother following a separation from the mother.
NOT AS GOOD. The distress behaviors still continue in the presence of the unrelated adult female monkeys, so it's harder to say that they are a "suitable substitute".