What Occurrence Defines the End of the Protostar Phase of a Star's

Multiple Choice

What occurrence defines the end of the protostar phase of a star's life and the start of the main-sequence phase?

A) Convection begins in its interior.
B) The star stops accreting mass from the interstellar cloud.
C) The star begins to expand to become a red supergiant.
D) The nuclear reactions in its core allow it to gain hydrostatic equilibrium.