The correct answer is The port ' s VLAN assignment on the switch because the symptoms indicate a Layer 2 segmentation issue. According to CompTIA Network+ (N10-009) troubleshooting methodology and VLAN concepts, if the IP configuration is correct and the TCP/IP stack is functioning, but the default gateway is unreachable, the issue often involves improper VLAN configuration.

In a switched environment, devices must be assigned to the correct VLAN to communicate with the appropriate default gateway interface (often configured on a router or Layer 3 switch). If the user’s switch port is assigned to the wrong VLAN, the workstation may have a valid IP address but will not be able to reach the gateway because it is logically separated into a different broadcast domain.

Option A is unlikely since the IP configuration is already confirmed correct. Option C is not relevant because the issue is localized to a specific user, not a widespread DHCP failure. Option D refers to switch management configuration, which does not affect user traffic routing to the gateway.

Therefore, verifying the switch port’s VLAN assignment is the most appropriate next troubleshooting step.