What must be ensured regarding out-of-service tracks adjacent to platforms?

The correct choice states that no trains should occupy the platform. This requirement is crucial for safety reasons when tracks adjacent to platforms are out of service. If a track is designated as out-of-service, it indicates that it is not safe for train operations, and any movement near that area could pose a risk to passengers. Keeping the platform free of trains ensures that passengers can safely embark and disembark, and reduces the chance of accidents occurring in the vicinity of the platform.

On the other hand, the other options do not address the fundamental safety and operational protocol concerning out-of-service tracks. For instance, having the track occupied by other trains contradicts the purpose of marking that track out-of-service, which is to keep trains away from that area. Similarly, notifying passengers of delays, while important for communication, does not directly relate to the safety of the platform area. Lastly, requiring all trains to maintain a complete stop does not specifically address the situation with adjacent out-of-service tracks, as it does not inherently prevent potential hazards associated with occupying the platform itself.