What Are "Thermal Pulses

What are "thermal pulses"?

A) These are the loops formed on the H-R diagram as the evolutionary tracks of low-mass stars move beyond the planetary nebula stage.
B) These are the pulses of radiation emitted during helium shell flashes.
C) This is another name for the high luminosity pulses of radiation emitted periodically by variable stars.
D) This is the burst of radiation given off during a nova.