Which event marks the beginning of a supernova

A) the onset of helium burning after a helium flash in a star with mass comparable to that of the Sun
B) the sudden outpouring of X-rays from a newly formed accretion disk
C) the sudden collapse of an iron core into a compact ball of neutrons
D) the beginning of neon burning in an extremely massive star
E) the expansion of a low-mass star into a red giant