The correct option is B I had not prepared lunch by noon.
While reducing a sentence to an adverb phrase, the subject and verb must be removed. In "It was noon", 'it' and 'was' is removed. 'By noon' is an adverb phrase that retains the meaning of the original sentence.
The given sentences must be reduced to a simple sentence. Option B is correct because it contains the relevant adverb phrase.
Option A does not contain an adverb phrase. 'For' is a coordinating conjunction that forms a compound sentence where the subject and verb are retained to form two independent clauses. Hence A is incorrect.
Option C is incorrect because it doesn't make sense. Moreover it does not contain an adverb phrase.
Option D is incorrect as it is a compound sentence, and the adverb phrase hasn't been used.