To answer this question, we need to understand the meaning of the word "prate".
Option A) Increase in the volume or intensity as in a musical passage - This option is incorrect because "prate" does not refer to an increase in volume or intensity in a musical passage.
Option B) Trace - This option is incorrect because "prate" does not mean trace.
Option C) (Of a fault or sin) forgivable - This option is incorrect because "prate" does not refer to the forgivability of a fault or sin.
Option D) Talk idly - This option is correct because "prate" means to talk idly or chatter on in a foolish or simple-minded way.
Therefore, the correct answer is D) talk idly.