Your answer to the question above would be A, that is, "a magical semicircle" is a noun phrase. Noun phrases are phrases which have a noun or a pronoun as head of the phrase together with any dependent words before or after the head. The noun head in this sentence is premodified by the qualitative adjective "magical".
B is not correct because it is an adjectival phrase, C stands for a verb phrase and, just the same as B, D is an adjectival phrase.