Final answer:
The first stage in the life cycle of a star is the nebula phase, where a star begins its life as a cloud of gas and dust. Following this phase, the nebula contracts to form a protostar, later entering the main sequence, and eventually evolves into a red giant as it ages.
Step-by-step explanation:
The first stage in the life cycle of a star is best described by option (c) Nebula. A nebula is a cloud of gas and dust in space, and it represents the very earliest phase when a star begins to form. After a nebula undergoes contraction due to its own gravity, it becomes a protostar, which is the precursor to the main sequence stage of a star's life. The sequence of a star's evolution goes from a nebula to a protostar, onto the main sequence where it spends the majority of its lifetime stably fusing hydrogen into helium, and then as the star depletes its hydrogen, it evolves into a red giant or supergiant before undergoing further transformations such as a supernova or becoming a white dwarf.
Different stars will progress through these stages at different rates, largely depending on their mass. Massive stars burn through their nuclear fuel more quickly, moving through the evolutionary stages faster than less massive stars. The life story of most stars begins with a nebula and over time, as they age, they pass through these stages, eventually moving off the main sequence toward becoming red giants or supergiants.