The correct answer is (d) Shaft
To explain I would say: The helicopter is an essential component that provides the vertical lift. The main rotor generates the lift whereas the tail rotor counteracts the torque thus eliminating yaw-motion. The rotor system consists of the hub on which all the blades are attached, it also has the mast. Together these three components make up the entire rotor system. The rotor system in turn is attached to the shaft through which the power is delivered.