The correct answer is:

D. VLAN based on MAC address

Explanation:
When users frequently change office locations and connect to the network from different switches, assigning them to a VLAN based on their MAC address ensures that they remain in the same VLAN regardless of the port or switch they connect to. Here's why:

VLAN based on MAC address:
Each user’s device has a unique MAC address, which can be used as a consistent identifier. By mapping specific MAC addresses to VLAN 10, users will always be assigned to VLAN 10, even if they change their physical location or connect to different ports/switches.
Why not the other options?
A. VLAN based on port:
VLAN assignment by port is static and tied to the physical location of the device. If users move to different switches or ports, the VLAN assignment would no longer be consistent.

B. VLAN based on subnet:
VLAN assignment based on subnet relies on IP addresses. If the network dynamically assigns IPs (e.g., via DHCP), users might not consistently belong to VLAN 10 when their IP changes or when they move to a different subnet.

C. VLAN based on protocol:
Protocol-based VLANs assign traffic to VLANs based on the Layer 3 protocol being used (e.g., IP, IPX). This method is not suitable for grouping users regardless of their network location....