What Event Occurs at the End of the Pre-Main-Sequence Stage

What event occurs at the end of the pre-main-sequence stage of a star's life?

A) The star explodes, forming a supernova remnant.
B) Gas is spun off from the star's equator, from which planets may form.
C) Nuclear reactions begin in the star's core, converting hydrogen into helium.
D) The star begins a long period of contraction in which gravitational energy is converted into heat.