The correct answer is D. Link aggregation. Link aggregation combines multiple physical switch links into one logical connection. This increases available bandwidth and can also add redundancy, all without replacing the switch hardware itself. If users are experiencing slow throughput and the goal is to improve capacity using existing equipment, bundling links is a common and effective solution.
The other options do not directly increase throughput. SVI configuration provides Layer 3 interfaces for VLANs, but it does not add link capacity. VLAN trunking allows multiple VLANs to traverse one connection, but that does not increase bandwidth by itself. 802.1Q tagging identifies VLAN membership on tagged frames; it is related to trunking, not throughput expansion.
In many enterprise environments, link aggregation is implemented through standards such as LACP. This lets several ports act together as one logical channel, which can help distribute traffic more efficiently across the bundled interfaces. For Network+ exam scenarios, when the requirement is to improve throughput using the current switch hardware rather than buying faster switches, link aggregation is the most appropriate answer.