Why is Pluto no longer a planet?

Correct option is B

Because it is not large enough to be considered as a planet.
In 2006, the International Astronomical Union (IAU) redefined the definition of a planet. According to the new definition, a planet must meet three criteria:
 It must be in orbit around the Sun.
 It must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
 It must have "cleared the neighborhood" around its orbit, meaning that no other objects of a similar size or larger are in its path.
Pluto meets the first two criteria, but it does not meet the third criterion. Pluto's orbit overlaps with the Kuiper Belt, a region of the solar system that contains many small icy bodies. Because Pluto has not cleared the Kuiper Belt, it is no longer considered a planet. It is now classified as a dwarf planet.