An attacker sniffs encrypted traffic from the network and is subsequently able to decrypt it. The attacker can now use which cryptanalytic technique to attempt to discover the encryption key?

A birthday attack exploits collision probabilities in hash functions and is not used to recover encryption keys from ciphertext-plaintext pairs.

A known plaintext attack is passive - the attacker already has static ciphertext-plaintext pairs but cannot choose or generate new ones, which is less powerful than the scenario described.

A meet-in-the-middle attack targets double-encryption schemes such as 2DES by attacking each encryption layer independently; it is not the appropriate technique when an attacker has active decryption capability.

A chosen ciphertext attack (CCA) is a cryptanalytic technique where the attacker selects arbitrary ciphertexts, submits them to a decryption oracle, and analyzes the resulting plaintexts to deduce information about the key. The scenario describes exactly this capability - the attacker has sniffed ciphertext and has the ability to decrypt it, giving them access to a decryption oracle they can exploit to work backward toward the secret key.