A company upgrades its wireless infrastructure with a 1Gbps backbone, new APs, and new tablets...

The most likely answer is D. Switch bandwidth is creating a bottleneck. Even though the company upgraded the wireless environment with newer access points and new tablets, wireless performance still depends on the capacity of the wired network that the APs connect back to. If multiple APs are sending traffic through a shared switch uplink or limited switch bandwidth, that wired segment can become the choke point, preventing users from seeing the benefit of the newer wireless equipment.

This kind of issue shows up often when organizations focus on upgrading the wireless edge but do not account for the aggregate traffic load feeding into the switching infrastructure. Faster wireless standards can increase the amount of traffic that reaches the switch, but if the switching path cannot handle that volume efficiently, end-user throughput will still feel unchanged.

The other options are less convincing. One AP having no connected devices does not explain why speeds on active clients are not improved. VLAN assignment problems would more likely cause access or segmentation issues rather than a general performance ceiling. Device congestion can hurt performance, but the information given points more strongly to the wired backhaul as the limiting factor.

In this scenario, the wireless upgrade is being held back by the switching side of the network, making switch bandwidth bottleneck the best answer.