What two physical processes balance each other to create the condition known as gravitational equilibrium in stars?

gravitational force and outward pressure

Explanation:

The stars are very massive stellar objects, so they have a strong gravitational force, which drives the star to contract itself, but also in stars there are nuclear reactions such as fusion of hydrogen and other elements, that releases energy and creates a pressure from the center to the star exterior, an outward pressure that goes against the gravitational force. So when a star is stable these two forces exist in equilibrium or in balance, in which the star does not collapse by gravity or disintegrate by its outward pressure.

i think the answer is a, hope this is right, that would be my best guess.

layers of the earth affect the waves and these effects are seen on instruiments such as seimometers