The Spanning Tree Protocol (STP) is the correct answer because it was specifically designed to prevent Layer 2 loops in networks with redundant paths. By determining the optimal spanning tree and disabling unnecessary paths, STP ensures that there is a single active path between two network nodes, thus preventing loops, while maintaining network redundancy. Link Aggregation Control Protocol (LACP) is incorrect because it is used to combine several network interfaces at the link layer to improve throughput and provide redundancy. PortFast is incorrect as it is a feature within STP that allows ports to skip the normal STP calculation and states, immediately transitioning to forwarding state; PortFast should only be enabled on ports connected to end devices. Root Bridge is a term related to STP that identifies the main switch in the layer 2 topology, but it is not a protocol itself.