A square threaded screw is used to lift a load W by applying a force F. Efficiency of square threaded screw is expressed as

The correct option is: The ratio of work done by W per revolution to work done by F per revolution.

Detailed Explanation:
The efficiency of any mechanical machine is fundamentally defined as the ratio of the useful work output to the total work input:
$$\eta =\frac{\text{Useful Work Output}}{\text{Work Input}}$$
For a square threaded screw used to lift a load:

1. Work Done by the Load (Work Output):
When the screw completes one full revolution, the load $W$ is lifted vertically by a distance equal to the lead or pitch ($p$) of the screw thread. Therefore, the useful work output per revolution is:
$${\text{Work Output}}_{\text{per revolution}}=W\times p$$
2. Work Done by the Applied Force (Work Input):
To turn the screw one complete revolution, the effort force $F$ is applied tangent to the screw thread. In one revolution, the point of application of this force travels a distance equal to the circumference of the mean diameter of the screw ($2\pi r$, where $r$ is the mean radius). Thus, the work input per revolution is:
$${\text{Work Input}}_{\text{per revolution}}=F\times 2\pi r$$
Therefore, substituting these values back into the efficiency formula, the efficiency of a square threaded screw is expressed as the ratio of the work done by the load $W$ per revolution to the work done by the force $F$ per revolution.