Why Does the Sun Brighten with Time

Why does the Sun brighten with time?

A) as time progresses, helium starts to fuse in the core, in addition to hydrogen, leading to an increase in brightness
B) as hydrogen is converted into helium in the core, the number of hydrogen nuclei decreases, decreasing the fusion rate. To maintain the balance with gravity pressing inward, the core compensates by shrinking and heating up
C) as time progresses, more and more heat is trapped inside the core, leading to an increase in brightness
D) as hydrogen is converted into helium in the core, the number of hydrogen nuclei decreases, decreasing the fusion rate. To maintain the balance with gravity pressing inward, the core compensates by expanding and cooling