(Which mechanism can be applied to protect the integrity of plaintext when using AES?

AES by itself is a symmetric block cipher that provides confidentiality, but not guaranteed integrity unless used in an authenticated mode. To protect integrity of the plaintext (ensuring it has not been altered), a Message Authentication Code (MAC) can be applied. In the classic Encrypt-then-MAC pattern, the sender encrypts the plaintext with AES and then computes a MAC (often HMAC-SHA-256 or CMAC-AES) over the ciphertext (and relevant headers). The receiver verifies the MAC before attempting decryption, preventing tampering and many padding-oracle style vulnerabilities. Alternatively, AES can be used in an AEAD mode like AES-GCM, which produces an authentication tag serving a similar purpose, but among the listed options the general integrity mechanism is “MAC.” RC4 is an unrelated stream cipher and does not provide integrity. RSA is asymmetric and not the standard integrity add-on for AES-encrypted bulk data. Kerberos is an authentication protocol and key distribution system, not a message integrity primitive. Therefore, to protect plaintext integrity when using AES, the correct mechanism is a Message Authentication Code.