What is a protostar?

A protostar is a stage in the formation of a star that occurs when a dense region of gas and dust within a nebula begins to collapse under its own gravity. During this process, the material gathers together to form a ball-shaped mass, which will eventually become a fully developed star once it reaches the appropriate temperature and pressure for nuclear fusion to begin.

At this stage, the protostar is not yet a fully formed star and has not ignited nuclear fusion in its core, which distinguishes it from the other options provided. A fully formed star would be in a later stage of stellar evolution, and a supernova refers to the explosive death of certain types of stars. A neutron star is a different end state of a massive star's life cycle, resulting from the core collapse after a supernova explosion. Thus, the definition of a protostar accurately reflects its place in the lifecycle of star formation within a nebula.