Answer:
b. red giant stage
Explanation:
Sun is a medium sized yellow star. It was born in solar nebula 4.5 billion years ago and moved into main sequence. Currently, sun is a main sequence star and has a lifetime of about 5 more billion years. Once the fuel runs out, the sun would expand to form a red giant and throw out majority of its mass. At the end, the core would form a white dwarf.
Thus, the next stage in the life cycle of a star is the red-giant stage.
