What is required after coupling with only one driver?

The correct option highlights the importance of conducting a full train preparation process after coupling with only one driver. This procedure is essential to ensure that all components are properly aligned and functioning as intended. A thorough train prep confirms that the train's systems, such as brakes, lights, and communication equipment, are operational, reducing the risk of issues during operation.

By coupling with only one driver, there may be a discrepancy in the functioning or readiness of other components that need to be assessed and prepared before the train can be safely operated. This process mitigates any potential hazards that could arise from inadequate preparation.

In contrast, the other choices involve important safety and operational procedures, but they do not specifically address the immediate necessity of ensuring the overall readiness of the train post-coupling. Therefore, while safety protocols, emergency briefings, and driver rotation are vital practices in train operations, the immediate requirement following a single driver coupling is to perform a comprehensive train preparation.