All inner planets, including Mercury, Venus, Earth, and Mars, share the characteristic of having rocky surfaces. Unlike outer planets, they do not have rings, and while some have moons, this is not a trait common to all of them. Therefore, the correct answer is C. rocky surfaces.
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The inner planets are defined by their rocky surfaces, distinguishing them from the outer planets. While some of them have moons, none have rings. Therefore, the characteristic common to all inner planets is their rocky surfaces. ;