The correct answer is D) make arrangements for simultaneously testing main and auxiliary steam stops with water and steam pressure. When preparing to hydrostatically test water-tube boilers, the regulations require that the main and auxiliary steam stops be tested simultaneously with both water and steam pressure. This ensures that all the boiler components, including the main and auxiliary steam stops, are properly functioning under the increased pressure of the hydrostatic test. The other options are incorrect because they do not address the specific requirement for testing the steam stops under both water and steam pressure.