🚘 The main source of energy for a star as it grows in size to become a red giant is ... ⚗️

Question

 The main source of energy for a star as it grows in size to become a red giant is ... 

 Answers:  

a. hydrogen fusion in the central core. 

b. helium fusion in the central core. 

c. hydrogen fusion in a shell surrounding the central core. (Correct Answer)

d. gravitational contraction.