The correct answer is: C. common base
The input impedance of a transistor connected in common base arrangement is the highest. This is because the emitter is connected to the ground, and the base is connected to the input signal. This means that the input signal is applied directly to the base-emitter junction, which has a high impedance.
The input impedance of a transistor connected in common emitter arrangement is lower than that of a common base arrangement. This is because the collector is connected to the output load, and the emitter is connected to the input signal. This means that the input signal is applied to a resistor divider, which has a lower impedance than the base-emitter junction.
The input impedance of a transistor connected in common collector arrangement is the lowest. This is because the collector is connected to the output load, and the base is connected to the ground. This means that the input signal is applied to a resistor, which has the lowest impedance of all the three arrangements.
In conclusion, the input impedance of a transistor connected in common base arrangement is the highest. This is because the emitter is connected to the ground, and the base is connected to the input signal. This means that the input signal is applied directly to the base-emitter junction, which has a high impedance.