The correct option is B hardly
The correct answer is Option B.
The correct option is the adverb that describes his manner of work on Sunday.
Option A-HARD as an adverb can only be used after the verb and not before it.
Option C-RATHER is used to indicate one's preference. Here the sentence conveys no such meaning of preference.
Option D-MUCH means a large amount and is used as a determiner (a word that determines the kind of reference a noun or nouns have). It cannot be used as an adverb to describe the manner of his work.
Hence these 3 options are not the correct answer.
Option B-HARDLY correctly describes the manner of his work. As an adverb of manner and an adverb ending in -ly, it can be suitably used before the verb.
Hence Option B is the correct option.