Lamp efficiency means higher light output for same power input

Steps:

• Both lamps consume 40W at 240V, so same electrical energy input.
• Efficiency for lamps is luminous efficiency: light energy output per watt of power.
• Higher efficiency for X means it converts more input energy to light than Y.
• Thus, X outputs more light energy while Y wastes more as heat.

Why D is correct:

• Luminous efficiency defines more efficient lamps as producing greater light energy for identical power input, per the formula: efficiency = (light output / electrical input) × 100%.

Why the others are wrong:

• A: Same 40W rating means identical energy use and running cost over time.
• B: Resistance R = V²/P; same V and P yield identical R for both.
• C: While X produces less heat, efficiency directly measures light output, not heat reduction.

Final answer: D