From the perspective of long-term survival, it was best for peasants

Frankly, I love this question. B is correct.

This question uses a very classical logic "if-then" + "Logical Opposition"

The form is:
If A, then B
Conclusion: if C, then Not B
Assumption: C is a logical opposition of A


ANALYZE THE STIMULUS:

Hypothesis: IF strong peasants in Czarist Russia (CR) go into army, THEN they will have shorter life spans.
Conclusion: IF peasants in CR are weak, THEN they will have a longer life span.
Assumption of this argument: "WEAK peasants" is a logical opposition of “STRONG peasants”. Is this correct? Nope, It's totally WRONG.

For example: what is the logical opposition of hot?. Most people say "cold". It's wrong. The correct logical opposition of "hot" is "NOT hot".

** This is the same flaw of this argument. The correct logical opposition of “STRONG peasants” is "NOT STRONG peasants". But the argument says "to live longer, peasants should be WEAK". Must they be weak? No, "Not strong" is enough, for instance "intermediate strong".


ANALYZE EACH ANSWER:

A) It assumes, without justification, that the only factor helping weak peasants survive longer was the fact that they were not enlisted into the army.
Wrong. A says: “not go into army” helps WEAK peasants live longer. The fact helps ONLY weak people? How about intermediate strong peasants? The main conclusion is: Not go into army will help peasants (in general) live longer. A may be a correct ans if it say: "the only factor helping WEAK peasants survive longer was the fact that they were not enlisted into the army"

B) It overlooks the possibility that there were peasants of intermediate strength who enjoyed survival benefits greater than those of either the strong or the weak.
Correct. B shows one case of the logical opposition of “strong peasants” ==> "Intermediate strong peasants". B states the argument’s flaw. The argument maintains that strong peasants will have shorter life, thus they should be weak to have longer life. That’s incorrect logic. You just need “NOT STRONG peasants” (for example: intermediate strong, little above weak, etc.) You DO NOT need “WEAK”.

C) It assumes, without justification, that all strong peasants and all weak peasants of the time faced the same prospects in life.
Wrong. Definitely out of scope. Nothing about “they have same file span”, the argument just compare “strong” vs “weak” ==> That’s incorrect as stated above.


TAKEAWAY:
Logical Opposition of "X" is "NOT X"


Hope you enjoy this question.