Which of the following does NOT represent one of the nine traditional planets in our solar system?

Proxima Centauri does not represent one of the nine traditional planets in our solar system because it is not a planet at all. Instead, Proxima Centauri is a red dwarf star located in the Alpha Centauri star system, which is the closest star system to our solar system. In contrast, the other choices—Pluto, Earth, and Mars—are all recognized as planets or former planets within our solar system. Pluto was classified as the ninth planet until 2006, when it was redefined as a "dwarf planet" by the International Astronomical Union. Earth and Mars are currently categorized as terrestrial planets that orbit the Sun, along with the other recognized planets in our solar system. Thus, Proxima Centauri is excluded from the category of traditional planets.