Final answer:
The two forces that must be in balance to keep a star in equilibrium are the gravitational force and nuclear fusion.
Step-by-step explanation:
The two forces that must be in balance to keep a star in equilibrium are the gravitational force and the nuclear fusion (option a).
Gravitational force is responsible for pulling matter together in a star, while nuclear fusion is the process by which stars release energy. These two forces must be balanced so that the star remains stable and does not collapse under its own gravity or explode from an uncontrolled fusion reaction.