Final answer:
Pluto was reclassified as a dwarf planet because it shares its orbital neighborhood with other icy Kuiper Belt Objects, not because of its mass or the size of its moon Charon. Its inability to clear its orbit of other debris disqualified it from planet status according to the International Astronomical Union's definition.
Step-by-step explanation:
The main reason that Pluto is no longer considered a planet is option (d): Pluto shares its orbital neighborhood with other icy Kuiper Belt Objects. This factor is what ultimately led the International Astronomical Union in 2006 to redefine the criteria for planet status, which Pluto did not meet due to its inability to clear its neighboring region of other debris. This is also illustrated by the fact that Pluto is part of a population of objects known as trans-neptunian objects, which includes several other icy bodies beyond Neptune's orbit.
It was once thought that Pluto was similar in mass to Earth, which led to it being classified as a terrestrial planet. However, the discovery of its moon Charon helped scientists determine that Pluto's mass was much smaller than Earth's. Additionally, other objects of similar size, such as Eris, and smaller, like Makemake, were found, which expanded the known diversity of the solar system's bodies.
Therefore, the most significant factor in reclassifying Pluto as a dwarf planet was its comparison and interaction with similar objects in its orbital vicinity, not its mass, the size of its moon, the eccentricity of its orbit, or the discovery of Eris being more massive than Pluto.