The correct answer is: C. Power in the load resistance
When the load resistance equals the generator resistance, the maximum power is delivered to the load. This is because the voltage across the load is equal to the source voltage, and the current through the load is equal to the maximum current that the source can deliver. The power in the load is then equal to the product of the voltage and the current, which is maximized when the voltage and the current are both at their maximum values.
Option A: Current. The current in the circuit is equal to the voltage divided by the resistance. When the load resistance equals the generator resistance, the current is equal to the maximum current that the source can deliver. However, the power in the load is not maximized when the current is at its maximum value.
Option B: Efficiency of the circuit. The efficiency of the circuit is equal to the power in the load divided by the power supplied by the source. When the load resistance equals the generator resistance, the efficiency of the circuit is equal to 1. However, the power in the load is not maximized when the efficiency is at its maximum value.
Option D: Voltage across the load resistance. The voltage across the load resistance is equal to the source voltage divided by the load resistance. When the load resistance equals the generator resistance, the voltage across the load is equal to the source voltage. However, the power in the load is not maximized when the voltage across the load is at its maximum value.