In 2006, the International Astronomical Union (IAU) reclassified which celestial body from a planet to a dwarf planet?

Explanation

Pluto was reclassified as a dwarf planet in 2006 because it did not meet the IAU's third criterion for a planet: it has not 'cleared its neighborhood' of other objects around its orbit. Ceres and Eris are also dwarf planets, but Pluto was the one reclassified from its previous status as a major planet.