What distinguishes the outer planets from the inner planets?

The outer planets, also known as the gas giants, are primarily distinguished by their gaseous composition, which includes large amounts of hydrogen and helium. Unlike the inner planets, which are rocky and smaller, the outer planets are much larger in size and lack solid surfaces. They are characterized by thick atmospheres and extensive systems of rings and moons. This composition results in their distinctive properties, such as their lower densities and the presence of strong winds and storms in their atmospheres.

While options mentioning size, proximity to the sun, and the number of moons highlight some aspects of the outer planets, they do not accurately define their main distinguishing feature, which is their gaseous nature. Thus, the primary factor that sets the outer planets apart from the inner planets is indeed their composition, making them predominantly gaseous bodies.