When a star burns up all the hydrogen in its core:
a) the core expands and cools while the surface of the star contracts and heats up to become a white dwarf.
b) it immediately starts burning the helium in its core.
c) it explodes in a supernova
d) the core contracts and heats up while the surface of the star expands and cools to become a red giant or supergiant.
e) none of the above, stars never run out of hydrogen.