Aperson who overeats at his or her family's house over the holidays it is most likely eating because of
a. healthy living
b. hunger
c. the need for energy
d. appetite

I would say D: appetite; 

appetite is a strong desire or liking for something. At Holidays there is typically a large assortment of food that maybe you don't eat on a regular basis, therefore would have a big appetite for them.

That is