The ratio of output work to input work expressed as a percent is the of a machine.

The correct answer to the question is efficiency i.e the ratio of output work to the input expressed as percent is called the efficiency of a machine.

EXPLANATION:

The efficiency of a machine gives the idea about the amount of energy conserved or lost in a process.

Let us consider a machine which is subjected to some amount of work done on it.

Let the work done on machine or the effort  is W.

The machine performs its work due to the effort and produces the desired output.

Let the output produced by the machine is W' .

Hence, the efficiency of the machine is calculated as -

                        Efficiency