Which switch sends BPDUs in an 802.1D STP network?

In an 802.1D Spanning Tree Protocol (STP) network, the switch that sends Bridge Protocol Data Units (BPDUs) is the root bridge. The root bridge is the central point of the spanning tree and is elected based on the lowest bridge ID, which is determined by the bridge priority and MAC address. Once the root bridge is established, it is responsible for generating and transmitting BPDUs to inform all other switches in the network of its presence and to help maintain the spanning tree topology. This process ensures that all switches have a consistent view of the network and can manage loops effectively.

The other options suggest conditions that do not accurately represent the STP operation. For instance, while all switches participate in the STP process, only the root bridge actively sends out BPDUs. Switches with the highest priority or lowest cost might influence the election of the root bridge but do not specifically relate to BPDU transmission.