The correct answer is Block ciphers encrypt data in fixed sized blocks while stream ciphers encrypt data one bit or one byte at a time.

Block ciphers operate on fixed sized blocks such as 64 or 128 bits and they transform each block with a symmetric key to produce ciphertext. When data is larger than a single block a mode of operation is used and padding or chaining is applied so the cipher can handle arbitrary length messages. Stream ciphers generate a keystream that is combined with the plaintext one bit or one byte at a time typically with an XOR operation and they are useful when low latency or bit level processing is required.

Cloud KMS is incorrect because it names a key management service rather than a classification of symmetric ciphers and the option does not describe the operational difference between cipher types.

Block ciphers function without using any cryptographic keys is incorrect because block ciphers are symmetric algorithms that require a secret key to encrypt and decrypt each block and without a key there is no meaningful encryption.

Block ciphers are exclusively employed in public key cryptography is incorrect because block ciphers are symmetric key algorithms and they are not part of public key cryptography which uses asymmetric algorithms like RSA or elliptic curve methods.

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