In what situation must spanning-tree be implemented?
A. when fast convergence is required for link failures
B. when redundant Layer 2 links, that are not part of a single EtherChannel or bundle, exist between distribution switches
C. when trunks need to extend multiple VLANs across access switches
D. when a VLAN spans access layer switches
D
https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Campus/HA_campus_DG/hacampusdg.html