What does a firewall primarily protect against?

A firewall primarily serves as a security device that monitors and controls incoming and outgoing network traffic based on predetermined security rules. Its main purpose is to establish a barrier between a trusted internal network and untrusted external networks, such as the internet, to protect systems from malicious attacks. This includes safeguarding against various types of threats like hackers attempting to exploit vulnerabilities, malware attempting to enter a system, and unauthorized access attempts. By filtering the data packets entering or leaving the network, firewalls play a critical role in maintaining the integrity and security of the network by preventing harmful traffic from causing potential damage.

In regard to the other choices, while unauthorized access to personal computers can be a concern related to firewalls, the primary function encompasses broader network protection against malicious attacks as well. Data transfer limits and cloud service interruptions are not relevant to the specific role of a firewall, as those issues relate more to data management and service availability rather than direct security against intrusion or attacks. Thus, the most accurate description of what a firewall protects against is indeed malicious attacks on networks.