This is a very good question!
I would personally not recommend to speak out the prepositions and articles while answering the short questions. Instead, as the instructions are given, I'd prefer answering it with one word.
Another reason for that is to avoid any confusion. For instance, if the question is: Which is the largest bird on the earth? Now, if you answer 'An ostrich,' you are giving all chances to software to take it as 'anostrich' which is not a word! I don't say that it happens, but why take risk when the instructions say that -one word or a few! I would go for 'ostrich' leaving no chance to mishear.