Which criterion does Pluto not satisfy to be classified as a planet?

Pluto is classified as a "dwarf planet" primarily because it does not satisfy the criterion of having "cleared its neighborhood" of smaller objects around its orbit. This means that a planet should be gravitationally dominant in its orbital area and not share its orbital zone with other bodies of comparable size, aside from its own moons.

In contrast, the other criteria are met by Pluto. It orbits the Sun, is indeed massive enough for its own gravity to make it round, and although its atmosphere is thin, it still has one. Therefore, option B is the distinguishing factor that prevents Pluto from being classified as a full-fledged planet, as it exists in a region of space that includes other celestial objects, such as other dwarf planets and debris in the Kuiper Belt. Hence, the inability to clear its orbit is the critical reason for its classification as a dwarf planet rather than a traditional planet.