Pluto is known as a dwarf planet because it is.
The correct option is C Very small
Pluto is a rocky ice planet which has an average surface temperature of −220∘C but the reason why it is considered as a dwarf planet is its size. The size of Pluto is very small when compared to earth. It is even smaller than the Earth's moon. It is because of this reason that Pluto is called a dwarf planet.