Boron is non-metallic in nature. It is extremely hard and black coloured solid. Due to a very strong crystalline lattice, boron has an unusually high melting point. The rest of the members are soft metals with a low melting point and high electrical conductivity.
Hence, Solid Boron has very strong crystalline lattice, so its melting point is unusually high in group 13 elements.
Both (A) and (R) are correct and (R) is the correct explanation of (A)