Question

At which point in the life cycle of a star does nuclear fusion begin?

Answer 1

I would say B : main sequence is the answer . this is the answer i believe because the star will increase in size and than shine brightly and when it's done , it will get smaller turning into  nebula , eventually exploding sometime around the last stage , but not the last stage of b , c, or d

i really hope that this helps you a lot.