When Does a Star Become a Main- Sequence Star

Multiple Choice

When does a star become a main- sequence star?

A) the instant when hydrogen fusion first begins in the star's core
B) when the rate of hydrogen fusion in the star's core is high enough to sustain gravitational equilibrium
C) when the protostar assembles from its parent molecular cloud
D) when a star becomes luminous enough to emit thermal radiation
E) when hydrogen fusion is occurring throughout the star's interior