Which statement about a single-acting cylinder is correct?

A single-acting cylinder is designed to perform work in one direction, typically through hydraulic or pneumatic force. The correct statement about this type of cylinder is that the retraction of the piston is usually achieved by a spring. In a single-acting cylinder, the pressurized fluid (air or hydraulic fluid) is applied to one side of the piston to extend it. When this force is removed, the spring helps retract the piston to its original position. This feature allows for a simpler design and operation compared to double-acting cylinders, which utilize fluid pressure to move the piston in both directions.

The other statements about the single-acting cylinder do not accurately describe its functionality. For instance, it does not operate on a double-acting principle, as this type would require the ability to exert force in both directions, which is characteristic of double-acting cylinders. The statement regarding movement in one direction is incomplete, as it implies there is no retraction mechanism, which there is in the form of a spring. Lastly, stating that external power is required for retraction is inaccurate since the spring mechanism provides the necessary force for this action without additional external power.