Efficiency as output-to-input power ratio

Steps:

• Define efficiency as the fraction of input energy converted to useful output.
• Identify input power as P_in and useful output as P_out (sound and light).
• Apply formula: efficiency = useful output power / input power.
• Compute: P_out / P_in, matching option A.

Why A is correct:

• Efficiency is defined by the formula η = P_out / P_in, the standard ratio for power conversion devices.

Why the others are wrong:

• B: Represents fractional power loss (waste heat), not efficiency.
• C: Inversely gives power amplification factor, irrelevant here.
• D: Ratios output to waste power, not a standard efficiency measure.

Final answer: A