When Does a Star Become a Main-Sequence Star

When does a star become a main-sequence star?

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