In my opinion, the correct answer is B) for Halloween // is decorated. Adverb phrases are extended adverbs or phrases that function like adverbs, modifying the sentences' predicates. They tell us how, when, where, why the verb action is performed. In this case, the predicate is is decorated. We can ask the mental question: why or what for is the house decorated? The answer to this question would be for Halloween. And that is our adverb phrase. It shouldn't be confused with an adjective phrase "down the street", which describes the subject "the house".