The correct answer is C) 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 flow of the fluid is unidirectional, meaning it can only pass through the valve in one predetermined direction. This is important in various applications, such as preventing the reverse flow of fluids, maintaining pressure, and protecting equipment from damage. The other answer choices are incorrect because they do not accurately describe the primary function of a check valve.