What is a protostar?

A protostar is indeed characterized as a phase when a star is in the process of formation, specifically during the free-fall collapse of the molecular cloud from which it is being formed. During this stage, the material in the cloud begins to condense under gravity, leading to an increase in temperature and pressure at the core, which marks the initial stages of stellar development. This is a crucial phase because the conditions that arise during this collapse ultimately determine whether the star will reach nuclear fusion and become a main-sequence star.

The choice indicating that a protostar is a stage when a star is fully formed is not accurate, as fully formed stars are already in the main sequence phase, where hydrogen fusion occurs in their cores. Similarly, a star that has completed its life cycle refers to evolved stars, such as red giants or supernova remnants, which bear no relation to the formation stages of a protostar. Lastly, a protostar has no connection with black holes; black holes are remnants of massive stars that have undergone supernova explosions and are at the end of a star's life cycle, unlike the early developmental phase of a protostar. Thus, the defining characteristic of a protostar is indeed its association with the free-fall collapse, which is