AES utilizes which block cipher design structure?

Select one:

a$.$ Public$-$key cryptography, relying on different keys for encryption and decryption.

b$.$ Substitution$-$permutation network, employing layers of S$-$boxes and permutation$-$boxes.

c$.$ Stream cipher, encrypting the message bit by bit with a keystream.

d$.$ Feistel cipher, similar to DES but with enhanced complexity.