Efficiency as the ratio of useful output to total input

Steps:

• Recall that efficiency quantifies how effectively a system uses energy without waste.
• Identify the standard formula: efficiency = (useful energy output / total energy input) × 100%.
• Compare this to the choices to find the matching definition.
• Eliminate options that include waste or invert the ratio.

Why C is correct:

• It matches the physics definition and formula for efficiency, which is the fraction of input energy converted to useful work.

Why the others are wrong:

• A inverts the ratio and ignores useful output, misrepresenting energy conservation.
• B excludes total input, focusing only on waste relative to output, which is undefined.
• D measures waste fraction, the opposite of efficiency (inefficiency).

Final answer: C