What Particular Feature of Stellar Behavior Is Associated with the Fact

What particular feature of stellar behavior is associated with the fact that a star is on the main sequence in the Hertzsprung-Russell diagram?

A) The star is generating internal energy by hydrogen fusion in its core.
B) The star is slowly shrinking, thereby releasing gravitational potential energy.
C) The star is generating energy by helium fusion in its core, having stopped hydrogen "burning."
D) The star has ceased nuclear "burning" and is simply cooling down by emitting radiation.