Which security control is most helpful in protecting against eavesdropping on wide area network $($WAN$)$ transmissions?

Applying filters on exterior Internet Protocol $($IP$)$ stateful firewalls

Deploying an intrusion detection system$/$intrusion prevention system $($IDS$/$IPS$)$

Blocking Transmission Control Protocol $($TCP$)$ synchronize $($SYN$)$ open connections

Encrypting transmissions with virtual private networks $($VPNs$)$