The correct answer is A. Virtual LAN (VLAN).

According to CompTIA A+ Core 1 220-1201 objectives under Networking:

VLANs are used to segment a physical network into multiple logical networks. By assigning the accounting department to a separate VLAN, users on other VLANs cannot access its network traffic without routing, thereby isolating sensitive departments while using the same physical switch.

VLANs improve security, reduce broadcast traffic, and allow network administrators to enforce access controls at the layer 2 level.

Why the other options are incorrect:

B. DHCP reservations: Ensure that specific devices always receive the same IP address, but do not restrict network access.

C. Content filtering: Controls web or application traffic based on content, not segmenting network access between departments.

D. Port security: Limits which devices can connect to a port but does not provide full network segmentation for an entire department.