(A 4): The function of a governor is to maintain the speed of an engine within specified limited whenever there is a variation of load.

(B 2): Flywheel is used to smoothen the fluctuation in speed within a cycle, which is related to turning moment diagram.

(C 1): Dunkerley method is used to determine critical speed of a shaft rotor system.

(D 3): D’Alemebert’s principle states that inertia forces and couples, and the external forces and torque on a body together give statical equilibrium.

Hence, the correct option is (D).