The correct answer would be B "Please put my letter in that mailbox." because a demonstrative adjective that refers to something far away would use words like that and those. But seeing as mailbox is not plural, that answer C would not make sense.
Huh??????? what you need help with
Buses, school buses, motorcycles, trucks, tractor-trailers, bicycles, mopeds, roller blades, pedestrians, delivery vehicles, taxicabs, etc.