Your answer was right, but I´m not sure if I understood your question. But I´ll give it a try: if there were a fifth option, say (e) would do, then there would be two possible answers, (a) and (e). What did you do with the brush? - He asked me what I had done... What will you do with the brush? - He asked me what I would do...
He asked me what I ......... with the paint brush. (a) did do
(b) had done (c) will do
(d) have done
I answered 'b'.
In case I hadn't done anything with the brush by the time 'he asked me', is 'would do' also possible, please?
Yes, it would be correct under those conditions. OR:
He asked me what I would have done with the paint brush if I had had no painting to do.
He asked me what I would have been doing with the paint brush in an hour's time if plans went accordingly.
Once you remove the restraints of the slots in the sentences, you can write almost anything (as long as it's grammatical).