Which of the following attacks allows an attacker to recover the key in an RC4 encrypted stream from a large number of messages in that stream?

A SYN flood attack is a denial-of-service technique that overwhelms a server by sending many TCP SYN packets without completing the handshake, and has no relation to stream cipher cryptanalysis or key recovery.

A rainbow table attack uses precomputed hash-to-plaintext mappings to crack password hashes, not to recover stream cipher encryption keys from captured traffic.

A zero-day attack exploits a previously unknown and unpatched software vulnerability, and is entirely unrelated to cryptanalytic attacks against the RC4 stream cipher algorithm.

The FMS attack, named after its discoverers Fluhrer, Mantin, and Shamir, targets a statistical weakness in RC4's key scheduling algorithm (KSA) where certain 'weak' initialization vectors (IVs) leak bits of the secret key. By capturing a sufficiently large number of messages encrypted with these weak IVs, an attacker can statistically reconstruct the full RC4 key. This attack was famously exploited against WEP-encrypted Wi-Fi networks.