Which planet is not classified as a terrestrial planet?

The correct answer identifies that Mars is not classified as a terrestrial planet. In actuality, Mars is indeed classified as a terrestrial planet. All the listed options, including Mercury, Venus, Earth, and Mars, are terrestrial planets, characterized by rocky surfaces and proximity to the Sun.

For the purpose of clarity, a terrestrial planet refers to those that are primarily composed of silicate rocks or metals and has a hard surface. There are four terrestrial planets in our solar system - Mercury, Venus, Earth, and Mars.

All the choices provided fall under the category of terrestrial planets. If the question intended to ask for a planet that is not terrestrial, it might refer instead to gas giants such as Jupiter or Saturn, which have significantly different compositions and environmental conditions compared to the terrestrial planets.