Which type of valve allows fluid to flow in one direction only?

A check valve is designed specifically to allow fluid to flow in one direction while preventing backflow. This function is crucial in many fluid systems to maintain the correct operation and safety of the system. By only permitting fluid to flow through the valve in the intended direction, a check valve prevents issues such as contamination and pressure fluctuations that could arise from reverse flow.

In comparison, a directional control valve can direct fluid flow but may change flow paths and does not inherently stop backflow. A flow control valve regulates the flow rate but does not limit direction. Meanwhile, a bypass valve serves to redirect flow around a component, which is not its primary function. The primary design feature of a check valve is to ensure unidirectional flow, making it the correct choice for this question.