What are the evolutionary stages of a star similar in size to the Sun?

The correct choice outlines the evolutionary stages of a star similar in size to the Sun, going through a well-established sequence: protostar, main sequence star, and then red giant.

Initially, a star forms from a cloud of gas and dust within a nebula. This collapsing material creates a protostar, a dense core where nuclear fusion has not yet begun. Once the core temperature becomes high enough for hydrogen fusion to occur, the star enters the main sequence phase, which is the longest and most stable part of its life cycle, during which it fuses hydrogen into helium in its core.

When the hydrogen fuel runs low, the star begins to expand and cool, entering the red giant phase. In this stage, it will fuse helium into heavier elements as it continues to evolve. Eventually, after exhausting its nuclear fuel, the star will shed its outer layers, leaving behind a white dwarf, but this specific evolutionary trajectory effectively showcases the transition through the protostar, main sequence, and red giant stages.

This sequence is characteristic of low to medium mass stars, like our Sun, making it the correct representation of their lifecycle.