The correct option is A Both the thieves
A noun phrase is a group of words containing a noun and functioning as a
subject, object or object of preposition. A noun phrase is often the
noun or pronoun itself. "Both the thieves" is a noun phrase because it contains a quantifier "both" that modifies the noun "thieves"; also, it is the subject of the given sentence. Hence option A is correct. "Thieves" is a noun, but it's a part of the noun phrase which is complete only with the quantifier. Hence "thieves" isn't the subject here, it isn't a noun phrase and option B is incorrect. "Have fled" is a verb phrase, made up of the helping verb "have" that determines the tense of a sentence, and the main verb "fled". It isn't a noun phrase and hence option C is incorrect. "With the jewels" is a prepositional phrase containing the preposition "with"; it's not a noun phrase and hence option D is incorrect.