The correct answer is: None of these.
When the water rises above ballast level, but below rail level in cause ways, train may not be permitted to run. This is because the water can cause the ballast to become unstable, which can lead to derailment.
The ballast is the layer of crushed stone that supports the rails. It is important that the ballast is kept dry and free of debris in order to provide a stable foundation for the rails. If the ballast becomes wet, it can become unstable and shift, which can cause the rails to move out of alignment. This can lead to derailment.
In order to prevent derailment, it is important to inspect the ballast regularly and to take steps to keep it dry. If the water level in a causeway rises above the ballast level, it is important to stop train traffic until the water level has receded.
The options A, B, and C are incorrect because they all state that train may be permitted to run when the water rises above ballast level, but below rail level in cause ways. This is not correct, as train may not be permitted to run in these conditions.