Turbine Efficiency Definition Steps:

• Identify input power as P1 (total power supplied to turbine).
• Identify output power as P2 (useful mechanical power delivered).
• Recall efficiency formula: η = (useful output power) / (input power).
• Compute η = P2 / P1; power loss PL = P1 - P2 is not in the efficiency ratio.

Why A is correct:

• Matches the standard definition of mechanical efficiency as output power divided by input power.

Why the others are wrong:

• B: P1/P2 exceeds 1 and inverts the efficiency ratio.
• C: PL/P1 represents fractional power loss, not efficiency.
• D: P1/PL is the reciprocal of fractional loss, unrelated to efficiency.

Final answer: A