The correct answer is: A. common emitter.
In a common emitter configuration, the input signal is applied to the base, the output signal is taken from the collector, and the emitter is connected to the ground. This configuration provides the highest power gain because the emitter is connected to the ground, which provides a low impedance path for the current to flow. This results in a higher output voltage and current, which gives the common emitter configuration the highest power gain.
In a common base configuration, the input signal is applied to the base, the output signal is taken from the collector, and the emitter is connected to the positive supply voltage. This configuration provides the highest current gain because the emitter is connected to the positive supply voltage, which provides a high impedance path for the current to flow. This results in a higher output current, which gives the common base configuration the highest current gain.
In a common collector configuration, the input signal is applied to the base, the output signal is taken from the collector, and the emitter is connected to the output signal. This configuration provides the highest input impedance because the emitter is connected to the output signal, which provides a high impedance path for the input signal to flow. This results in a lower input current, which gives the common collector configuration the highest input impedance.
Therefore, the power gain in a transistor connected in common emitter arrangement is the highest.