In what situation must spanning-tree be implemented?

A.
when first hop redundancy protocol exists with redundant Layer 2 links between distribution switches

B.
when a VLAN spans access layer switches to support business applications

C.
when trunks need to extend multiple VLANs across access switches

D.
when it is necessary to speed up network convergence in case of link failure

Explanation:
https://www.safaribooksonline.com/library/view/foundation-learning-guide/9780132652933/ch02.html
You may need to implement STP for several reasons:
• When a VLAN spans access layer switches to support business applications.