The correct answer is A. 10.
A medical relief van shall be dispatched within 10 minutes from double exit siding. This is because the double exit siding is a designated area where trains can stop and unload passengers in case of an emergency. The medical relief van is equipped with medical supplies and personnel who can provide first aid and other medical assistance to passengers who are injured or ill.
Option B is incorrect because it is too long. A medical relief van should be dispatched as soon as possible after an emergency occurs. Option C is incorrect because it is too short. A medical relief van should have enough time to gather its supplies and personnel before departing for the scene of the emergency. Option D is incorrect because it is not a valid option.