Which of the following is the correct way to form the superlative of the adverb sadly?

A. sadly

B. sadliest

C. more sadliest

D. most sadly

The correct way to form the superlative of the adverb sadly is D. most sadly.

Positive: sadly; comparative: more sadly; superlative: most sadly

For example: He played this song most sadly of all.

B is superlative of the adjective sad, but not adverb sadly.