A gas water heater is 75% efficient. The energy output used to heat the water is 4.6 kWh.

Efficiency is useful output energy divided by input energy. So the energy supplied to the heater is 4.6 kWh ÷ 0.75 ≈ 6.1 kWh. The correct choice is 6.1 kWh.

AI-written and checked against the verified answer.