Option 1 -> "Between" requires two or more entities (between A and B), but only one friend is mentioned.
Option 2 -> "Beside" means "next to" or "at the side of" - fits perfectly for sitting next to a friend.
Option 3 -> "Besides" means "in addition to" or "apart from" - doesn't fit the context of physical seating.
Option 4 -> "Beneath" means "under" or "below" - inappropriate for a seating arrangement.
Hence, Option 2: beside -> "Beside" correctly indicates the boy wants to sit next to (at the side of) his best friend, which is the natural way to describe seating positions in a classroom -> correct