A Star Is Observed to Be on or Close to the Main-Sequence

A star is observed to be on or close to the main-sequence region of the Hertzsprung-Russell diagram. What particular feature of stellar behavior can be inferred by this observation?

A) The star is generating internal energy by hydrogen fusion.
B) The star is generating energy by helium fusion, having stopped hydrogen "burning."
C) The star is slowly shrinking and thereby releasing gravitational potential energy as it slides down the main sequence from top left to bottom right across the H-R diagram.
D) Stars that have reached the main sequence have ceased nuclear "burning" and are simply cooling down by emitting radiation.