In Spanning Tree Protocol (STP), the root bridge is determined by the lowest Bridge ID, which is a combination of bridge priority and the MAC address. In the given scenario, the bridge IDs are: SW1 = 24596 0018.184e.3c00, SW2 = 28692 004a.13e9.6900, SW3 = 32788 0022.55cf.dd00, and SW4 = 64000 0041.396d.690f. The priority of SW1 is 24596, which is the lowest among all switches listed. Therefore, SW1 will become the root of the spanning tree for VLAN 20.