Final answer:
Earth is the planet with the greatest mean density, having a density of approximately 5.5 g/cm³, which is higher than all other planets in the Solar System.
Step-by-step explanation:
The planet with the greatest mean density is Earth. Earth's density is approximately 5.5 g/cm³, which is significantly higher than Jupiter's average density of 1.3 g/cm³, Neptune's density of 1.6 g/cm³, and even Mercury's density of 5.4 g/cm³. Although Mercury has a high density for its size, it is still not as dense as Earth. The terrestrial planets, like Earth and Mercury, tend to have higher densities than the gas giants like Jupiter and the ice giants like Neptune because they are composed of heavier materials such as rock and metal, rather than gas. Therefore, the correct answer to the student's question is B) Earth.