The correct answer is D) Permits flow in one direction only. A check valve is a type of valve that allows fluid (liquid or gas) to flow in only one direction, preventing backflow. The purpose of a check valve is to ensure that the fluid can only move in the desired direction, not in the opposite direction. This is an important safety and efficiency feature in many fluid systems, as it helps to maintain proper flow and prevent potential problems caused by reverse flow. The other answer choices are incorrect because they do not fully capture the purpose of a check valve. Option A) is incorrect because a check valve does not simply pass air but not liquid - it permits flow in one direction regardless of the fluid type. Option B) is incorrect because a check valve does not regulate liquid flow, but rather allows flow in one direction only. Option C) is incorrect because a check valve can pass both liquid and air, as long as the flow is in the permitted direction.