What Physical Mechanism Is Responsible for the Oscillations in Brightness

What physical mechanism is responsible for the oscillations in brightness that a Cepheid variable star undergoes?

A) the fusion rate in the star's core varies periodically, changing the energy generation rate
B) the amount of ionized helium in the star's outer layers vary, changing the opacity
C) the fusion rate in the shell burning hydrogen varies, changing the energy generation rate
D) the star's outer layers cycle between convection and radiative transport, changing the temperature