A firewall is a critical part of network security that monitors and controls incoming and outgoing network traffic based on predetermined security rules. Its function is to block unauthorized access while permitting legitimate communication, thereby serving as a barrier between trusted internal networks and untrusted external networks.
Option A: Incorrect-phishing is handled by user awareness and email filters, not firewalls alone.
Option B: Incorrect-malware detection is performed by antivirus/antimalware tools, not firewalls.
Option D: Incorrect-encryption keys are managed by cryptographic systems, not firewalls.
Thus, the function of a firewall is to control unauthorized access.
Reference:
WGU Information Technology Management - Network Security Fundamentals, section on Firewalls.