What Major Physical Process Is Taking Place Inside Stars That

What major physical process is taking place inside stars that are on the main sequence in the Hertzsprung-Russell diagram?

A) Hydrogen is being converted to helium in their cores.
B) Hydrogen is being converted to helium in a shell around the helium-rich core.
C) Helium is being converted to carbon in their cores.
D) The gas is contracting gravitationally without nuclear reactions taking place.