What must be done before testing the emergency application capability from the rear of the train?

Answer: B — Close the angle cock ahead of the last car.

Before testing the emergency application capability from the rear of the train, it is crucial to close the angle cock ahead of the last car. This action ensures that the air supply to the rear of the train is isolated appropriately, allowing for a clear and effective test of the emergency braking system. By closing the angle cock, any air leaks or flow that could affect the testing process are minimized, ensuring an accurate assessment of how the emergency application operates in real scenarios. It also prevents any inadvertently released air from affecting the integrity of the test results, allowing for proper emergency response training and capabilities assessment.