The answer is Pluto.
Pluto is a dwarf planet, while the other four options are planets. Pluto was originally classified as a planet when it was discovered in 1930, but it was reclassified as a dwarf planet in 2006.
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System. It is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts and polar ice caps of Earth.
The Sun is a star, a giant ball of hot plasma held together by gravity. It is the center of our solar system and the source of almost all of the energy on Earth.
Saturn is the sixth planet from the Sun and the second-largest planet in the Solar System. It is a gas giant with an average radius of about 9.5 times that of Earth. Saturn is known for its extensive ring system, which is composed of countless small particles ranging in size from dust to small moons.
Mercury is the smallest planet in the Solar System and the closest to the Sun. It is a terrestrial planet with a solid surface. Mercury has no atmosphere, and its surface is heavily cratered.