Which of the following causes a single acting cylinder to retract?

A single acting cylinder is designed to move in one direction under fluid pressure, typically extending when pressurized fluid fills the cylinder. However, to retract, it requires a different mechanism. When fluid is not supplied to the retraction side, a spring is often implemented on the unpressurized side of the piston. This spring force pushes the piston back to its original position after the pressurized fluid is released.

In this case, the choice mentions spring force acting on the unpressurized side of the piston. This is the correct explanation for how a single acting cylinder retracts. When the pressurized fluid is removed, the spring force comes into play, driving the piston back to the home position. This mechanism is essential for the design and functionality of single acting cylinders, illustrating the reliance on spring action for retraction rather than fluid power alone.

The other options incorrectly describe the retraction process. Fluid pressure acting on the pressurized side wouldn't facilitate retraction since it would keep the piston extended. Similarly, fluid pressure on both sides doesn’t constitute a retraction mechanism; it would generally lead to equal force balance with no movement. Lastly, blocking flow from the pressurized side could potentially trap the fluid but does not inherently cause the cylinder to retract; rather