The Sun moves off the main sequence as the Hydrogen in its core becomes exhausted. Directly after the Hydrogen in the core is exhausted, how does the Sun create energy from fusion?

a$.$ The Sun no longer creates energy from fusion and becomes a white dwarf.

b$.$ The Sun continues to burn Hydrogen in the core because of an increase in temperature.

Q$.$ The Sun burns Hydrogen in a shell around the core.

d$.$ The Sun burn Helium into Carbon and Oxygen in the core.