Which button must be pressed to finalize the arming of an ETD?

The correct answer is the COMMUNICATIONS TEST/ARM button. This button is specifically designed to both test the communication systems of the electronic train detection (ETD) unit and arm it for operation. When you press this button, it completes the necessary steps to ensure that the ETD is ready for use, confirming that it is in an operational state and capable of transmitting relevant data.

The functionality of the COMMUNICATIONS TEST/ARM button is critical in ensuring the safety and efficiency of train operations. Arming the ETD is an essential step in preparing the system to monitor and interact with the train’s air brake system, and this button is the key to initiating that process.

Other buttons mentioned, like the TEST button or the ARM button, may have functions related to different aspects of the system but do not serve the dual purpose of both arming and testing communications. The SET button may refer to other configurations but does not pertain to the finalization of arming the ETD specifically. Therefore, the COMMUNICATIONS TEST/ARM button is the appropriate choice for this task.