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