Firewalls are classic examples of technical controls as they use technology to restrict or permit network traffic based on security policies. Technical controls are implemented through hardware or software solutions.
Security guards (A) and fences (C) are physical controls, while policies (B) are administrative controls.
Technical controls like firewalls are fundamental in securing network perimeters and endpoints as covered in the Security Architecture domain#6:Chapter 3 CompTIA Security+ Study Guide#.