Which of the following characteristics do all of the outer planets have in common? A. They lack moons. B. They are small and dense. C. They have liquid cores. D. They are mostly made up of hydrogen and helium.

I think the correct answer from the choices listed above is option D. Outer planets are mostly made up of gases and are huge in size. These gases are hydrogen and helium. These outer planets are Saturn, Jupiter, Uranus, and Neptune. Hope this answers the question.