Which characteristic is essential for a check valve to function correctly?

A check valve is designed specifically to permit fluid flow in one direction while preventing backflow, ensuring that the system operates efficiently and safely. This characteristic is essential because it helps maintain the intended flow path and pressure levels within the system, which is crucial for applications like refrigeration. If a check valve were to allow flow in both directions, it could lead to potential system failures, contamination, or inefficiencies.

While opacity, bidirectional flow, and pressure increase can be relevant to other components within a system, they do not pertain to the primary function of a check valve. The key focus of a check valve's design and operation is to facilitate unidirectional flow, thereby enforcing the proper hydraulic dynamics necessary for effective system performance.