Which of the following terms best describes an early stage of star formation?

The term that best describes an early stage of star formation is "protostar." A protostar is formed from the gravitational collapse of a region within a nebula, where dust and gas come together to create a dense core. As material continues to accumulate, the temperature and pressure in the core increase, leading to conditions favorable for nuclear fusion to eventually start. This process marks the transition from a cloud of gas and dust (nebula) to a fully fledged star.

In contrast, a black hole is a region of space with a gravitational pull so strong that nothing, not even light, can escape from it; this occurs much later in the life cycle of massive stars after they have gone through supernova explosions. A supernova is an explosive end to a massive star's life cycle, releasing an enormous amount of energy but is not related to the early stages of star formation. Throughout this lifecycle, the nebula serves as the initial material, but it does not itself represent the early stage of star formation; rather, it's the environment from which protostars arise. Thus, "protostar" is the most accurate term to indicate an early stage of star formation.