Which statement about the efficiency of a machine is true?

• A. Efficiency is the ratio of energy input to useful energy output.
• B. Efficiency is the ratio of useful energy output to energy input, with no units.
• C. Efficiency is the loss of energy due to friction.
• D. Efficiency is the ratio of useful energy output to total energy input × 100%.

Answer: (D)

Efficiency measures how much of the energy you put in actually becomes useful output. It is defined as the useful energy output divided by the total energy input, multiplied by 100 to give a percentage. This captures why machines can’t be perfectly efficient: some energy is always lost to heat, friction, sound, and other forms. So the best description is that efficiency equals useful energy output over total energy input, times 100%. The other statements miss the correct direction of the ratio or describe energy losses rather than the efficiency definition, or omit the percentage form.