When a Star Leaves the Main Sequence and Expands Toward

When a star leaves the main sequence and expands toward the red giant region, what is happening inside the star?

A) Helium burning is taking place in a spherical shell just outside the core; the core itself is almost pure carbon and oxygen.
B) Hydrogen burning is taking place in a spherical shell just outside the core; the core itself is almost pure helium.
C) Hydrogen burning is taking place in a spherical shell just outside the core; the core has not yet started thermonuclear reactions and is still mostly hydrogen.
D) Helium is being converted into carbon and oxygen in the core.