Which of the following serves as a barrier to unauthorized network access?

A firewall serves as a barrier to unauthorized network access by monitoring and controlling incoming and outgoing network traffic based on predetermined security rules. Firewalls create a protective barrier between a trusted internal network and untrusted external networks, such as the internet. By watching the data packets attempting to enter or leave the network, firewalls can block harmful traffic and prevent potential attacks from external sources.

While encryption and hashing are important security measures, they serve different purposes. Encryption protects data by converting it into a format that can only be read by someone with the correct decryption key, ensuring confidentiality, but does not directly block unauthorized access to the network. Hashing provides data integrity by verifying that data has not been altered, but it does not function as a barrier to access. Thus, the primary function of a firewall in preventing unauthorized network access makes it the correct answer.