What is the next step in the sun's life cycle after it has used up its hydrogen fuel. a. red giant b. nebula c. supernova d. super giant

The correct answer is - A. Red Giant.

The Red Giant faze of a star/sun occurs when all of the hydrogen has been used by it. In this faze, the star starts to expand, thus becomes much bigger, and takes a reddish coloration, and in the faze after this one, it does the opposite, it retracts, and becomes much smaller.

Our Sun will experience the same in the distant future, when it will start to expand, consume the inner planets, probably even the Earth itself too.

The Red Star faze of a star, in essence, is marking the beginning of the end of the star, or rather, it is marking its dying out.