I think this question is not really GMAT-style and the answer choices are...weird. IMO you'll not see something like that on the actual test.
But that's OK, let's take a look.
Conclusion: Younger children lack the ability to see what another being's perspective is.
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a) Children of all ages that are affected by autism also describe what is directly in front of them when asked what the bear sees.
It doesn't affect the conclusion. The author is concerned with an ability to see from someone else's perspective in two different age groups: young children and older children. Children with autism are a special case and form another group that we're not interested in. It can be true, that autistic kids AND younger children (without autism) lack this ability, and the argument wouldn't be affected.
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b) Researchers ran another experiment where they took a tall narrow glass and a short wide glass and poured an equal amount of liquid back and forth between them. Older children would say that the amount of liquid stayed the same. Younger children would say that there was more in the tall narrow glass.
The "glass experiment" doesn't include "seeing from someone else's perspective" thing and tests different abilities. Hence, we can't use it to weaken author's conclusion.
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c) Younger children often did not understand the question being asked; when they were asked the more simple question "what is in front of the bear?", they would answer the same as the older children.
Choice C suggests that there is another factor that may have caused the differences between younger and older children. If younger children simply didn't understand the question, we can't conclude that they gave the wrong answer because they "lack the ability to see what another being's perspective is". This definitely weakens author's conclusion.
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d) Male and female children that were older would answer the question in generally the same manner.
Unnecessary differentiation in the older group. We are concerned with younger vs older children differences, not with male vs female.
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e) Older children are less likely to play with teddy bears every day than are younger children.
I can't tell how experience in playing with teddy bears would affect one's ability to put oneself in someone else's shoes. Irrelevant.
The winner is C!