Answer should be D.

Option D explains the premise of vervet monkeys having two different types of calls, AND the varying responses according to the calls.

Now some people may argue that Option C provides a logical reasoning for the vervet monkeys having two different sets of calls. However we need to assume that Vervet monkeys have 2 different reactions to each alarm calls (which would be D). Even if vervet monkeys have 2 alarm calls, if the reaction is the same, there would be no point in having a different set of calls.

Hope I am right.

My take is option D.
I am not sure about this.
In my 2 cents,different alarm calls need to mean something,i.e. difference countermeasures.
Otherwise,why not just one uniform alert !?

The correct answer choice is (D). The situation in the stimulus is that
vervet monkeys use different calls depending on where predators come from.
The correct answer must explain why the calls are different (again, difference
versus similarity is an issue). Note that the stimulus does not contain a true
paradox, just an odd situation that is presented without explanation. Answer
choice (A): This answer states that vervet monkeys vary the calls in order to
indicate the number of predators, but the answer does not explain why
different calls are used for land versus air predators. This answer is attractive
because it shows that different calls can be used to indicate different things,
but it is wrong because it does not explain the behavior of the monkeys as
described in the stimulus. Answer choice (B): This answer addresses only
land-based predators and does not explain the difference described in the
stimulus. Answer choice (C): This answer states that the predators using land
attacks are different from the predators using air attacks, but this information
does not explain why vervet monkeys use different calls to indicate that fact.
Answer choice (D): This is the correct answer. Because vervet monkeys react
to predators in different ways, they would need to know if the predator was
coming by land or air. Hence, the different calls are used to tell the monkeys
whether they should climb trees or dive into the foliage. Since this answer
explains the behavior of vervet monkeys, this answer is correct. Answer
choice (E): The diet of selected predators of vervet monkeys is irrelevant and
does not help explain why vervet monkeys use different calls depending on
the direction of the attack.
_________________