What was as remarkable as the development of the compact disc has been the use of the new technology to revitalize, in better sound than was ever before possible, some of the classic recorded performances of the pre-LP era.

So to maintain parallelism , we should use the development . Narrows down to A / C .
IMO C.

"what was as remarkable as" is wordy and awkward. Go with the more direct version in C.

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Explanation by Stuart: https://www.beatthegmat.com/question-t7939.html
This question is mostly about parallelism.

(a) has non-parallel tenses. For (a) to make sense, it would have to begin "what is as remarkable as", since the "remarkability" refers to the present (i.e. it's remarkable now). (b), (d) and (e) all make the same error.

Only (C) doesn't make the same mistake, therefore (c) must be correct.

Another issue with (d) and (e) is the incorrect idiom "less remarkable as" - "less remarkable than" is idiomatically correct. Another reason to eliminate (b) is that "as the developing compact disc" makes no sense in this sentence ("the development of the compact disc" has a completely different meaning and we want to avoid changing the meaning of the original sentence).

(c) uses the idiomatically correct (and parallel) "No less remarkable than the X has been the Y"