Answer: RC4 is the correct answer for not being an example of a block cipher, as it is a stream cipher. Stream ciphers are more malleable than block ciphers, which encrypt data in fixed blocks while stream ciphers encrypt one byte at a time. Both block ciphers and stream ciphers are types of symmetric key ciphers. Although block ciphers are more commonly used in cybersecurity, stream ciphers are not as well studied. AES is often mistaken for a block cipher, but it is actually a stream cipher. The non-reversible block cipher, also known as a Pseudo Random Permutation (PRP), is a synonym for block cipher.