A transformer with efficiency 80% works at 4 kW and?

Here, η=80%
Let the input power be Pin and the output power be Pout.
Given that the efficiency η = 80% = 0.8
Output power Pout = 4 kW = 4000 W
Efficiency η = Pout / Pin
0.8 = 4000 / Pin
Pin = 4000 / 0.8 = 5000 W
The input voltage is not given, so we assume it is 230V.
Input current Iin = Pin / Vin = 5000 / 230 = 21.74 A
Let the output voltage be Vout and the output current be Iout.
Output power Pout = Vout * Iout = 4000 W
Assuming the output voltage is 115V:
Iout = Pout / Vout = 4000 / 115 = 34.78 A
The efficiency of a transformer is defined as the ratio of the output power to the input power. The problem statement is incomplete and doesn't specify the input and output voltages, making it impossible to uniquely determine the input and output currents. To find the input and output currents, we need information about both the input and output voltage. The provided options suggest an additional piece of information is missing from the original question. The solution above demonstrates a possible approach with assumed voltages.