Gamma. See the Related Link.
The Greek alphabet has no equivalent of the letter c.
Depends on what type of c. C as in cylinder is Sigma.C as in cat is Kappa.C as in chorus is Chi.There isn't much difference between Chi and Kappa.
ητα (η,Η) and ταυ (τ,Τ)
All 26 letters of the Latin alphabet are missing from the greek alphabet, because it is an entirely different writing system.
alpha