The correct answer is A. high.
The efficiency of a machine is a measure of how much of the input energy is converted into useful output energy. A machine with a high efficiency will convert more of the input energy into useful output energy, while a machine with a low efficiency will convert less of the input energy into useful output energy.
A machine with a high efficiency will require less input energy to produce the same amount of output energy, which will reduce the operating cost of the machine.
A machine with a low efficiency will require more input energy to produce the same amount of output energy, which will increase the operating cost of the machine.
Therefore, the efficiency of a machine should be as high as possible to reduce the operating cost.
Option B is incorrect because a machine with a low efficiency will require more input energy to produce the same amount of output energy, which will increase the operating cost of the machine.
Option C is incorrect because a machine with a high efficiency will require less input energy to produce the same amount of output energy, which will reduce the operating cost of the machine.
Option D is incorrect because the efficiency of a machine should be as high as possible to reduce the operating cost.