During the formation of a star, the contraction stops once
select one:
a. hydrogen fusion becomes the dominant energy source.
b. the star collapses into a black hole.
c. the star collapses into a white dwarf.
d. helium fusion becomes the dominant energy source.

The correct option is option a. hydrogen fusion becomes the dominant energy source.

Explanation:

Options b and c are things that happen to a star when it is nearing its end of life, being becoming a black hole or a white dwarf the final destiny of the star.

Option d happens at the maturity of the star, when enough helium accumulates (from hydrogen fusion) in its core in order to start fusing.

Option a is correct because when hydrogen fusion becomes the dominant energy source the star stops contracting due to inward pressure from gravitational collapse (which allows the initial concentration of mass that forms a proto star) starts being in equilibrium with the outward thermal pressure due to the energy liberated in form of heat by the fusion of hydrogen in the core.