Answer: D. Outward pressure and inward gravity
Explanation:
In the core of a star, hydrogen atoms fuse to form Helium atoms. This nuclear fusion reaction generates immense amount of heat and pressure. The pressure pushes the atmosphere of the star outwards. This pressure is balanced by the gravity of the mass pulling itself inside. Thus, an equilibrium is maintained. If a dis-balance is created, the star expands when there is excess pressure or collapses under its own weight.
