The correct answer is Spanning Tree Protocol (STP). STP is crucial for preventing loops in a network by detecting and disabling redundant paths, ensuring a loop-free topology. Whenever STP senses a loop, it blocks redundant links and only allows necessary paths to be active to prevent broadcast storms and possible data collisions. Port Mirroring, RSTP, and Quality of Service (QoS) are false traps. RSTP is just a faster version of STP but doesn't inherently offer advantages specific to the scenario described. Port Mirroring is used for monitoring and not for loop prevention, whereas QoS prioritizes traffic and also isn't used to avoid loops.