There is no underlined phrase in the sentence. However, we can assume it is "under my hiking boots" because of the answer options provided.
Answer:
The underlined phrase in the sentence is an:
b. adverb phrase
Step-by-step explanation:
An adverb phrase or adverbial phrase functions as an adverb in the sentence. In other words, it modifies a verb, an adjective, or another adverb. Generally adverbs and adverbial sentences give us information concerning the where, when, how, or why the action happened.
In the sentence "I wear heavy wool socks under my hiking boots," the phrase "under my hiking boots" is an example of an adverbial phrase. It indicates place, that is, where the action happens. Where do you wear your wool socks? Under my hiking boots.