What is a primary characteristic of terrestrial planets?

The primary characteristic of terrestrial planets is their composition, which includes a high proportion of heavy metals and rocky materials. Terrestrial planets, such as Mercury, Venus, Earth, and Mars, are defined by their solid surfaces and are closer to the sun compared to gas giants. The substantial presence of heavy metals like iron and nickel contributes to the overall density and geological features of these planets. This composition leads to a variety of surface conditions and structures, such as mountains, valleys, and craters, distinguishing them from gas giants, which are primarily composed of lighter gases and do not have solid surfaces.

In contrast, the other options suggest characteristics that do not apply to terrestrial planets: gas planets tend to be larger and composed mainly of gases, and while some terrestrial planets have water, it is not abundant on all of them. Additionally, terrestrial planets are typically located within the inner solar system, rather than orbiting far from the sun.