The following things are called celestial bodies () A. Lightning B. Rainbow in the sky C. Nebula D. Sun

A celestial bodies are celestial bodies observed in the sky and existing outside the earth's atmosphere, such as stars, planets, satellites, comets, meteoroids, nebulae, galaxies, etc., collectively referred to as "celestial bodies"; As a whole, the earth is also a celestial body; When an artificial object is launched from the earth into outer space, it becomes an artificial celestial body. Lightning is a weather phenomenon in the earth's atmosphere, so it is not a celestial body, so it does not meet the meaning of the question.

According to the analysis just now, the rainbow in the sky is a physical phenomenon in the earth's atmosphere, not a celestial body, so it does not meet the problem.

C, nebulae are substances in the universe, and the most basic celestial bodies in the universe belong to celestial bodies, so they are correct.

D, the solar system is composed of several celestial bodies, not celestial bodies, so it does not meet the meaning of the question.

So choose: C.