Answer: (2.) The subset of the US population with Medicare , and (3.) not all Medicare drug plans are noun phrases.
Explanation: A noun phrase is a group of words that has a noun or indefinite pronoun as its head. However, it can also include premodifiers and/or postmodifiers. A noun phrase can play different functions in a sentence: for example, it may act as a subject or as a direct object. 2) and 3) are noun phrases because in both of them there is a noun acting as the head. In the case of 2), the head is "subset", while in 3), the head is "plans". Moreover, in 2), the head is being premodified and postmodified, while in 3), "plans" is only being premodified. In contrast, 1) is a verb phrase and 4), a prepositional phrase.
