When Does a Protostar Become a Main-Sequence Star

Multiple Choice

When does a protostar become a main-sequence star?

A) when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space
B) when a piece of a molecular cloud first begins to contract into a star
C) when it becomes luminous enough to emit thermal radiation
D) at the instant that the first hydrogen fusion reactions occur in the protostar's core