The Mechanism That Gives Rise to the Phenomenon of the Nova

The mechanism that gives rise to the phenomenon of the nova is

A) the impact and subsequent explosion of a large comet nucleus on a star's surface.
B) material falling into a black hole and being condensed to the point where a thermonuclear explosion is produced.
C) the complete disintegration of a massive star due to a runaway thermonuclear explosion in the star's interior.
D) matter from a companion star falling onto a white dwarf in a close binary system, eventually causing a nuclear explosion on the dwarf's surface.