A) Digital signatures using MD5 hashing: MD5 is a cryptographic hash function, but it's now considered cryptographically broken and unsuitable for security purposes like digital signatures. Collisions (where two different inputs produce the same hash) can be easily found, meaning a malicious actor could potentially alter data without detection. While digital signatures can ensure integrity, using MD5 makes this method insecure.
B) Cyclic Redundancy Check (CRC): CRC is an error-detection code commonly used to detect accidental changes to data during transmission. It's very effective at detecting common transmission errors but is not designed to protect against deliberate manipulation. It's a good check for accidental corruption, but not for malicious tampering.
C) Base64 encoding of the transmitted data: Base64 is an encoding scheme that translates binary data into an ASCII string format. It's primarily used to transmit binary data over text-based channels (like email). It does not provide any data integrity checks or security features. It just makes the data transmitable.
D) SHA-256 hash with message authentication code (HMAC): SHA-256 is a strong cryptographic hash function. HMAC uses a secret key in conjunction with the hash function. This means that only someone with the secret key can generate the correct HMAC. If the data is altered during transmission, the calculated HMAC at the receiving end will not match the transmitted HMAC, thus revealing the tampering. This method provides both integrity and authentication.