Nuclear Fusion Reactions That Convert Helium into Carbon and Oxygen

Nuclear fusion reactions that convert helium into carbon and oxygen in the central core of a star occur during which phase of a star's life?

A) in the red giant stage, before the helium flash
B) during and immediately after the (first) red giant or supergiant stage
C) during the protostar stage
D) after the main-sequence phase, before the star becomes a red giant