Correct choice is (a) Flap motion
Easiest explanation: The hinged blade allows rigid body rotation about the hinge point which is acted upon by restoring moment due to centrifugal force acting on the rotating blade. For the hinge that lies in the plane of the rotor disk, there’s an out-of-plane deflection of the blades produced. This is known as flap motion. When the deflection of blade along vertical hinge arises, it is known as lag motion.