Which networking device is commonly used to enforce security policies on traffic flowing in and out of a network?

The correct answer is the firewall because it is specifically designed to monitor and control incoming and outgoing network traffic based on predetermined security rules. Firewalls act as a barrier between a trusted internal network and untrusted external networks, such as the internet, ensuring that only authorized traffic is allowed through. They can inspect packets and determine whether to allow or block traffic based on security policies, making them essential for maintaining the security of a network.

In contrast, other devices like switches primarily function to direct traffic within a local area network (LAN) without undertaking security filtering. Routers connect different networks and can provide basic access control but are not specialized for security enforcement. Load balancers distribute network or application traffic across multiple servers to optimize resource use and performance, rather than focusing on security. Thus, the firewall is the device best suited for enforcing security policies on network traffic.