Answer:
The grammatical structure of "To open the door" in the sentence "Will someone be here soon to open the door?" is a (D) Infinitive phrase.
Infinitive phrases are formed by the word "to" followed by the base form of a verb (in this case, "open"). They can function as nouns, adjectives, or adverbs in a sentence. In this sentence, the infinitive phrase "to open the door" acts as an adverbial phrase, providing information about the purpose or intention of someone being here soon.