Step 1: Think of the network's front door.
Every network that connects to the outside world, like the internet, needs a checkpoint that inspects what is trying to come in or go out, much like a security guard at a building entrance.
Step 2: See what a firewall actually checks.
A firewall examines each piece of network traffic against a set of security rules, such as source and destination address, port number, or protocol type, and decides whether to allow or block it.
Step 3: Rule out the other options.
It is not a physical boundary, not an operating system, and not a browsing tool, its entire purpose is enforcing access control so that only legitimate, authorized traffic gets through.
\[ \boxed{\text{a system designed to prevent unauthorized access}} \]