How does Spanning Tree Protocol detect and monitor loops in networks?

A. STP detects and monitors BPDUs being received on multiple interfaces.
B. STP detects and monitors normal traffic frames being received on multiple interfaces.
C. STP detects and monitors CDP frames being received on multiple interfaces.
D. STP detects and monitors access ports in the same VLAN.

A. STP monitors all interfaces for BPDUs, which carry switches’ identities. When it sees the same switch ID in BPDUs on multiple interfaces, a redundant link is detected.