BPDU Guard performs the function of shutting down a port if a BPDU is seen on that port. Therefore, the correct answer from the provided options is:
Shuts down a port if a BPDU is seen on that port.
BPDU Guard is typically used on ports configured with PortFast to prevent loops by ensuring that only end devices (like PCs and printers) are connected to the port. If a BPDU is detected, it indicates that a switch has been connected to the port, which could potentially create a loop in the network, so the port is shut down to prevent that.