Question

what is a protostar? a contracting mass of gas which represents an early stage in the formation of a star

Explanation:

To determine the correct definition of a protostar, we recall astronomical concepts. A protostar is the early stage in star formation, consisting of a contracting mass of gas (before nuclear fusion starts). The orange option matches this definition. The purple option likely refers to a main - sequence star (related to HR diagram), and the green option's partial text doesn't fit.

Answer:

A. (The orange option: A contracting mass of gas which represents an early stage in the formation of a star)