Final answer:
A firewall is the correct answer; it serves as a barrier that blocks unauthorized access to a computer system, using predetermined rules to filter out harmful traffic.
Step-by-step explanation:
The device that uses hardware or software to create a barrier to prevent unwanted messages or instructions from entering a computer system is known as a firewall. A firewall meticulously analyzes incoming traffic based on pre-established rules and filters out potentially harmful data that could compromise the security of the system. It serves as a gatekeeper, deciding which data packets can enter or leave the network. Firewalls can be implemented in either hardware or software, or a combination of both, and are essential for protecting a network from various types of cyber threats such as hacking, viruses, and worms.