Question

when stars are in the main sequence, fusing hydrogen into helium, and are neither expanding nor contracting, they are said to be... options: red hypergiants, dying, white dwarfs, in hydrostatic equilibrium

Explanation:

When stars are in the main sequence, they fuse hydrogen into helium and are in a state where gravitational forces and internal pressure balance, known as hydrostatic equilibrium. The other options (red hypergiants, dying, white dwarfs) don't describe the main - sequence star state. Red hypergiants are a later evolutionary stage, "dying" is not the correct term for main - sequence stars, and white dwarfs are also a later stage.

Answer:

in hydrostatic equilibrium