Step 1: Plan of attack: We will describe the friend in simple words first: a person who values old, historic places. Then we look for the option whose meaning is the same.
Step 2: Test choice 1. Greedy: the offer gave something to the family, so it is not about taking. Mismatch.
Step 3: Test choice 2. Opportunist: an opportunist uses a situation for his own benefit. The museum plan was for the history of the place. Mismatch.
Step 4: Test choice 3. Romanist: this word has no connection with preserving a house. Mismatch.
Step 5: Test choice 4. Antiquarian: a lover of antiques and old places. This is the same as our description. Match.
Step 6: Put the answer back and re-read: Read the question again in full: Choose the best option to complete the sentence: From the offer made by the narrator's friend, it's evident that the friend is- Now place the choice "an Antiquarian" into it. The sentence stays natural, it agrees with the passage, and it adds no new fact of its own. Each of the other three choices fails at least one of these checks, as the tests above showed. A choice that fails even one check cannot be the best answer.
Final Answer: Our description matches only option 4, so the answer is an antiquarian. \[\boxed{4}\]
