What role do S$-$boxes play in S$-$DES encryption?

Select one:

a$.$ They serve as decryption tables, converting ciphertext back into plaintext during decryption.

b$.$ They act as random number generators to inject unpredictability into the encryption process.

c$.$ They perform non$-$linear substitutions on specific bit blocks to obscure the plaintext pattern.

d$.$ They provide secure storage for the encryption key, preventing unauthorized access.