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 hash collision probability and is used against hashing algorithms, not as a technique to discover symmetric encryption keys from captured ciphertext.

A known plaintext attack requires the attacker to possess plaintext-ciphertext pairs they did not actively choose - having the ability to decrypt any chosen ciphertext is a stronger, distinct capability classified as a chosen ciphertext attack.

A meet in the middle attack targets double-encryption schemes such as 2DES by attacking the encryption and decryption ends simultaneously, which does not apply to the single-cipher scenario described.

A chosen ciphertext attack occurs when an attacker can select arbitrary ciphertexts and obtain their corresponding plaintexts - in this scenario, the attacker sniffed ciphertext and can already decrypt it, effectively giving them a decryption oracle. By carefully selecting ciphertexts and analyzing the resulting plaintexts, the attacker can gather information to deduce the underlying encryption key.