Which Solar System object is this?
Sorry, but the сorrect answer is:
Ceres is the largest body in the "Asteroid Belt" that lies between Mars and Jupiter, containing about a third of the total mass of the entire Asteroid Belt.
Ceres was discovered in 1801 and for about 50 years was considered a "planet" before it went the way of Pluto and got kicked out of the planetary "club". Interestingly, when Pluto got demoted from "Planet" to "Dwarf Planet", Ceres actually got a promotion from "Asteroid" to "Dwarf Planet", although it is still considered an asteroid by many scientists. It is, at this time, the only celestial body to have a dual-classification: Dwarf Planet-Asteroid.