Refer to the exhibit.

Switch D should be the VLAN 10 primary root, Switch C should be the VLAN 10 secondary root, and link A should remain a Layer 2 trunk. The requirements say VLANs span multiple access switches and all VLANs are trunked on all uplinks. In that design, the distribution interswitch link must remain Layer 2 so the VLAN can exist consistently across the access layer. For fastest and most deterministic Rapid PVST+ convergence, the spanning-tree root bridge should align with the active first-hop gateway for the VLAN. In the exhibit, Switch D is the active gateway for VLAN 10, so making Switch D the primary STP root avoids unnecessary traffic tromboning and keeps the forwarding path optimal. Switch C becomes the secondary root for resiliency. A Layer 3 routed interdistribution link is a best practice when VLANs are not stretched, but it contradicts the stated requirement that VLANs span access switches. Reference topics: Rapid PVST+, STP root placement, FHRP alignment, Layer 2 campus distribution, VLAN spanning design.