When a Star First Begins the Long Path Toward Becoming

When a star first begins the long path toward becoming a red giant, a layer of hydrogen around the core begins to undergo fusion. If this layer was too cold to do fusion throughout the main sequence stage, why is it suddenly warm enough?

A) as the star expands, all the layers heat up
B) the core is collapsing under its own weight and heating up from the compression; this heats the next layer up
C) the heat comes from the fusion of carbon in the core, which starts right away
D) the heat comes from the outer layers of the star, which are much hotter than the core
E) this is an unsolved problem in astronomy, but something must be heating that layer, since we observe red giants out there