What Does the Term "Hydrostatic Equilibrium" Mean in Reference to Stars

What does the term "hydrostatic equilibrium" mean in reference to stars?

A) The pressure throughout a star is constant.
B) The pressure within a star is sufficient to cause the star to expand at a constant rate.
C) The gravitational force within a star is sufficient to cause the star to collapse inward at a steady rate.
D) Each layer within a star is in balance with respect to pressure and gravity.