Answer:
Option (D)
Step-by-step explanation:
A dynamic equilibrium is the state in which the net force on the body is zero but the body is moving.
For option (a), A man is standing without moving, it means the man is in static equilibrium.
For option (b), a bicycle accelerating forward with acceleration, it means it is not in dynamic equilibrium.
For option (c), a car driving in circle with a constant speed, it means the direction of speed is changing, so the acceleration is there and the car is not in dynamic equilibrium.
For option (d), a motorcycle with a constant velocity, it means the net force is zero and the motorcycle is in dynamic equilibrium.