The correct answer is D. Mercury
Step-by-step explanation:
A dwarf planet is an astronomical object that resembles a planet because it orbits a star, but it is not considered a regular planet as they are too small and do not behave as dominant gravitational bodies like planets. In our solar system, astronomers have identified different dwarf planets that include Pluto that is in the Kuiper belt, Eris that was discovered in 2005 and Charon that is considered by some as a dwarf planet and by other as the satellite of Pluto.
However, Mercury is not considered as a dwarf planet because despite being the smallest from all the planets in the Solar System its size and features make it a planet, indeed Mercury is the first planet in the Solar System. Thus, the one that is not a dwarf planet is Mercury.