Vervet monkeys use different alarm calls to warn
each other of nearby predators, depending on
whether the danger comes from land or from the air.
Which one of the following, if true, contributes most
to an explanation of the behavior of vervet monkeys
described above?
(A) By varying the pitch of its alarm call, a vervet
monkey can indicate the number of predators
approaching. --- The issue is not regarding number of predators.
(B) Different land-based predators are responsible
for different numbers of vervet monkey
deaths. ---out of context
(C) No predators that pose a danger to vervet
monkeys can attack both from land and from
the air. ---- Again, irrelevant.
(D) Vervet monkeys avoid land-based predators by
climbing trees but avoid predation from the
air by diving into foliage. --- Correct. Since velvet monkeys have defense mechanisms for both land predators and air predators, so they warn differently for the different predators. This helps them keep themselves safe.
(E) Certain land-based predators feed only on
vervet monkeys, whereas every predator that
attacks vervet monkeys from the air feeds on
many different animals.--- Out of scope again.

Why cant C be right?

C indicates that predators will either attach from land or from air. so it indicates that monkeys use different sounds to let their group know who is going to attack so that they can take necessary action.