The correct answer is B. It is an adverb clause modifying "set"
Explanation:
The main grammatical function of adverb clauses such as the one in the sentence (Before dinner is ready) is to modify or describe other elements in the sentence, especially the main verb of the sentence or another adverb. Indeed, in the sentence presented, the adverb clause modifies the main verb "set" because the words "before dinner is ready" describe the specific time the table needs to be "set". According to this, the second option is the correct answer.
