An aid is a tool, not a method. A chart does not teach by itself; the teacher's question about the chart teaches. The exam question is therefore never "is this aid good?" but "which sense does it use, and which activity does it support?"
Worked example: a teacher plays a recorded speech for a language class. That is an audio aid: it serves listening. Showing the same speech as a silent chart would not serve the goal, because the goal needs sound. Match the aid to the sense the learning needs.