What is the primary feature that sets a Hill cipher apart from other substitution ciphers?

Select one:

a$.$ The encryption and decoding processes rely on rigorous mathematical formulas.

b$.$ It moves letters within the alphabet by a predetermined amount of positions.

c$.$ To create ciphertext, it multiplies characters in plaintext by a square matrix.

d$.$ It substitutes a different letter for each one according to a predetermined keyword.