Final answer:
The sentence "The young gentleman is engaged _____ Miss Brown." is correctly completed with option B) 'to.' The young gentleman is engaged to Miss Brown.
Step-by-step explanation:
The correct answer to complete the sentence "The young gentleman is engaged _____ Miss Brown." is B) to. Therefore, the completed sentence should read: "The young gentleman is engaged to Miss Brown." In English, when expressing that someone is promised to another in marriage, the preposition 'to' is used. This is a standard usage in formal English and is commonly seen in announcements of engagements.