Answer: Option (D) is the correct answer.
Step-by-step explanation:
When a force is applied by a person to a system then this force is known as input force.
It is also known as initial force that needs to be applied in order to start a machine.
For example, when a piston in a system is pressed inwards by a person then an input force is applied to the system.
Thus, we can conclude that an input force the force a person applies to a simple machine.