This is an assumption question, so we'll need to think particularly about how the writer moves from the premises to the conclusion, which in particular requires identifying the conclusion explicitly.
Effectively the author of this argument concludes that diplomatic/legal language is stilted because it's so important to avoid misunderstandings in these domains. So even without looking at the answers, I think there's got to be an assumption that connected stilted language with clarity and complete lack of ambiguity. I'll still keep my eyes peeled for other assumptions though - it's not possible to anticipate every possible version of a correct answer, after all.
(A) looks good - it's similar to what I had anticipated above, though a little more toned day, basically saying that stilted language/non-literary language is clearer than literary language. Let's keep it but check out the others.
(B) is out of scope - we don't care about importance per se - nothing the author writes suggests that works of literary merit are not important.
(C) looks alright - but do we really need this? This almost seems circular in assuming what is effectively the conclusion. I might have kept this if I hadn't already seen (A) - as it is, let's get rid of it.
(D) is out of scope - we don't care what is of interest to anyone in this question.
(E) is overly general and out of scope - cautiousness might be linked to non-literary quality writing, but this in turn requires an assumption in addition to the one in the answer choice - so it's out.
Our answer is (A). Make sure to look back at this one if it gave you trouble. It's important to understand why (C) is not the answer.