Explanation:
BPDU Guard prevents switching loops in the case of a switch being connected to a PortFast interface. PortFast is used for ports that connect to host systems, such as workstations and printers, and allows the port to immediately enter a forwarding state. This bypasses the normal 30-second delay that Spanning Tree Protocol would normally use to determine if a switch has been connected to the port. Implementing BPDU Guard will disable the port if a switch is connected and a BPDU is received.

Rapid Spanning Tree Protocol (RSTP) is incorrect because this is an enhanced Spanning Tree standard that operates on the Data Link layer of the OSI model. RSTP was not designed to protect PortFast ports. PortFast and BPDU Guard are supported by RSTP, but they not required or configured by default.

Root Guard is incorrect because it is used to protect the root bridge placement in the Spanning Tree, not to protect PortFast ports.

Per-VLAN Spanning Tree (PVST) is incorrect because this is an implementation of Spanning Tree (the default protocol for Cisco switches), and was not designed to protect PortFast ports. PortFast and BPDU Guard are supported by RSTP, but are not required, and must be configured manually.