Final answer:
The correct answer to the question is D) Mercury, which is the smallest terrestrial planet with a diameter of 4878 kilometers, less than half of Earth's diameter, and has a mass one-eighteenth that of Earth.
Step-by-step explanation:
The smallest terrestrial planet is Mercury. Mercury is notable for being the smallest planet in our solar system, with a diameter of 4878 kilometers, which is less than half that of Earth. This makes it significantly smaller than Mars, Neptune, or Ganymede (which is a moon, not a planet).
In our solar system, the term terrestrial planets refers to the four planets closest to the Sun - Mercury through Mars. They are composed mainly of rock and metal and have solid surfaces. Mercury's mass is one-eighteenth that of Earth, and its density is 5.4 g/cm³, indicating a composition different from that of the Moon.
When we compare Mercury with Neptune, we should note that Neptune is not a terrestrial planet but a jovian, or gas giant, and significantly larger. Ganymede, being a moon (specifically, Jupiter's largest moon), doesn't qualify as a planet, and Mars, while also a terrestrial planet, is larger than Mercury.