The reader can infer jack wants to be called Merridew because:
A) He hates the name Jack.
B) He wants to be seen as an authority figure.
C) It is a custom for the oldest boy to be referred to by his last name.
D) There are several other kids named Jack on the island, and he doesn't want to confuse people.

im just here for the points

explanation:

lemme actually be useful and say i believe the answer is the first one.

are you kidding? from that description of two mansions, you can't figure out which one is bleak and desolate? maybe you should consider looking those words up in a dictionary, if you don't know what they mean already.