The correct answer is (a) Plot of power coefficient as a function of thrust coefficient
To explain I would say: Plot of power coefficient \(\frac {C_p}{σ}\) as a function of thrust coefficient \(\frac {C_T}{σ}\) is known as rotor polar. There is no loss of profile power for an ideal rotor and there is minimum induced loss. Due to profile power loss, the rotor polar for a real rotor is at an offset compared to the ideal polar, and power increases faster with thrust coefficient due to the larger induced power.