Question

Why is Pluto being the last planet out of the nine (9) planets referred to as DWARF PLANET?

Answer 1

That's an easy one: "Because it has not cleared the neighborhood around its orbit, Pluto is considered a dwarf planet. It orbits in a disc-like zone beyond the orbit of Neptune called the Kuiper belt, a distant region populated with frozen bodies left over from the solar system's formation."

From: www.nationalgeographic.com/science/space/solar-system/dwarf-planets-pluto-ceres/