Final answer:
The Spanning Tree Protocol (STP) is the only standard among the options that supports a single root bridge, resulting in traffic from all VLANs flowing over the same path.
Step-by-step explanation:
The spanning tree standard that supports only one root bridge, so that traffic from all VLANs flows over the same path, is the Spanning Tree Protocol (STP). Unlike STP, Rapid Spanning Tree Protocol (RSTP), Multiple Spanning Tree Protocol (MSTP), and Per-VLAN Spanning Tree (PVST) allow for a more complex network structure where multiple paths can be used based on different instances of Spanning Tree.
STP was designed to prevent network loops by creating a single-spanning tree that spans all switches in a network, which results in having only one active path between any two network devices. The singular nature of STP makes it simpler but less flexible compared to its advanced versions like RSTP, which is faster, and MSTP and PVST, which are VLAN-aware and allow for multiple spanning trees.