Ans: D.Nebula

Exp: Nebula is the dense cloud of gases mainly Helium, Hydrogen and dust. They occur at primary stages & then from them galaxies are formed. It is a cloud of gas and dust in outer space, visible in the night sky either as an indistinct bright patch or as a dark silhouette against other luminous matter.

A nebula is a giant cloud of dust and gas in space. Some nebulae (more than one nebula) come from the gas and dust thrown out by the explosion of a dying star, such as a supernova. Other nebulae are regions where new stars are beginning to form.