Answer: D. a nebula
Explanation:
Nebula is known as stellar nursery. It is composed of clouds of dust and gases from which stars are born. This cloud of dust and gases gravitates and shrinks to form a core of a star. When right temperature and pressure is reached, nuclear fusion kicks to produce heat and light which results in birth of a star. Astronomers think that solar system was formed from solar nebula. After the birth of the star, the rest of the material accreted to form the planets, asteroids and moons etc.