At What Point in Its Evolution Does a Protostar Stop

At what point in its evolution does a protostar stop shrinking and stabilize into a star?

A) when nuclear processes generate enough energy and internal pressure to resist gravitational contraction
B) when gravitational contraction has heated up the gas to the point where radiation pressure opposes gravity for the first time
C) when the protostar has spun off enough of its matter and is spinning fast enough that centrifugal force opposes the gravitational contraction
D) when nuclear reactions end in the star's core