The Most Important Development in the Interior of a Star

The most important development in the interior of a star of moderate mass that leads to the red giant phase is:

A) a runaway nuclear furnace and eventual explosion in the star's interior, destroying the star and leaving only a rapidly expanding shell of gas.
B) the cessation of hydrogen "burning" in the core, leading to core contraction and overall star expansion.
C) expansion of the whole star, leading to lower temperatures throughout the star and the turnoff of the nuclear furnace in the core.
D) contraction of the whole star, which quenches the nuclear furnace in the core and leads to cooling of the surface.