Question

what is a nebula?
the core of a massive star
a cloud of gas and dust in space
a swirling disk with a lot of gravity
the debris remaining after a supernova

Explanation:

A nebula is defined as a cloud of gas and dust in space. The core of a massive star is not a nebula. A swirling disk with a lot of gravity is more related to accretion disks around black holes or protostars. The debris remaining after a supernova is supernova remnant, which is a type of nebula but not the general definition. The most general and correct description is a cloud of gas and dust in space.

Answer:

a cloud of gas and dust in space