Answer :
"Both troops reproduced at the same rate, but more monkeys in troop #1 were being killed every year. Certain traits, such as avoiding predators by spending more time in trees, can provide individual organisms with a survival advantage in certain environments. In this case, spending more time in trees allowed the monkeys in the second troop to avoid predators better, so more of them were able to reproduce" (Source: Study Island).
THE ANSWER IS
Because members of the second troop were better able to avoid predators, more of them were able to reproduce.