A cloud storage provider discovers that an unauthorized party obtained a complete backup of encrypted...

The correct answer is Ciphertext-only attack. CEH cryptography coverage defines this attack as a situation in which the attacker has access only to encrypted data and not to the corresponding plaintext. The attacker attempts to derive useful information, recover plaintext patterns, or eventually infer the key by analyzing the ciphertext itself. That is exactly what the question describes: the adversary obtained encrypted database backups but not the keys and not the original plaintext records, and the specialist is considering whether the attacker is examining the encrypted output in isolation for structural clues or repetition. Known-plaintext attacks require access to both plaintext and matching ciphertext. Chosen-plaintext attacks involve the attacker selecting input to be encrypted, while chosen-ciphertext attacks involve selecting ciphertext for decryption attempts. None of those fit the scenario. CEH materials emphasize that ciphertext-only analysis is the most basic cryptanalytic model because it assumes the least attacker knowledge apart from the encrypted material itself. Since the exposure here is limited to intercepted ciphertext with no confirmed access to plaintext or decryption capability, the best match is a ciphertext-only attack.