Even though apes are the only nonhuman creatures able to learn human

The conclusion of this argument is that only humans have philosophical thought (which I'll refer to as "philosophy" from now on to type less!). Why is it that only humans have philosophy?

It's because apes are the only nonhuman animals to learn human language, and they haven't used it to ask a philosophical question.

Strange argument! Let's see if we can find a gap. I accept that apes are the only ones to learn our language and that they haven't asked any deep questions yet, but does it mean only humans have philosophy? For one, perhaps those apes just weren't compelled to speak their mind. Similarly, perhaps dolphins have philosophy, but express it in their own dolphin language.

We're looking for a necessary assumption, and (C) is one as it addresses that second gap. If we negate it, and assume that philosophy could be expressed in nonhuman languages, then the argument doesn't make sense since the premises are all about what's been done (or hasn't) in human language.

As for the wrong answers:

(A) is tricky. We actually learned that apes could learn human language, so (A) is just trying to take-down a premise!

(B) is tempting, but "thinking" is too broad. We're interested in philisophical thinking. Furthermore, whether apes are capable is not actually the issue, it's whether they do it.

(D) is tempting, but we're not interested in the relative difficulty of the tasks. Even if speaking were harder than thinking (in human language), we still could have apes be able to speak and not be able to think.

(E) is similar to (D) in that it compares how difficult various tasks are. It's irrelevant how hard these tasks are.