Efficiency as work output over heat input in isobaric expansion

Steps:

• Define efficiency η as useful work W divided by heat supplied q: η = W/q.
• Calculate work for constant-pressure expansion: W = p(V2 - V1).
• Substitute into efficiency: η = p(V2 - V1)/q.
• Compare to options: this is choice C.

Why C is correct:

• Matches thermodynamic definition of efficiency for heat engines, where isobaric work is pΔV from the first law.

Why the others are wrong:

• A: Uses initial volume pV1, ignoring expansion work ΔV.
• B: Adds irrelevant V2/V1 ratio and omits pressure p.
• D: Lacks pressure p, yielding incorrect units and value for work.

Final answer: C