In the sentence "Does the captain want us to lower the sails before we enter the harbor?" the phrase "to lower the sails" is a (D) Infinitive phrase.
Infinitive phrases are formed by the word "to" followed by the base form of a verb (in this case, "lower"). They can function as nouns, adjectives, or adverbs in a sentence. In this sentence, the infinitive phrase "to lower the sails" acts as a noun phrase and serves as the direct object of the verb "want." It answers the question "what does the captain want?" by specifying the action that the captain desires, which is the act of lowering the sails.