The correct answer is B) compressed air. The air cylinder bottles in survival craft, such as lifeboats and life rafts, should be refilled with compressed air. This is in accordance with the regulations set by the US Coast Guard for the maintenance and equipment of survival craft. Compressed air is used because it provides the necessary breathable air for the occupants of the survival craft in an emergency situation, without the risk of using pure oxygen which could be dangerous. The other options are incorrect because oxygen (A) is not the suitable gas for refilling the air cylinders, nitrogen (C) does not provide breathable air, and a mixture of nitrogen and oxygen (D) is not the standard requirement for survival craft air cylinders.