Which of the following objects is in dynamic equilibrium? a - a man standing in one place without moving b- a bicycle accelerating forward at 1.25 m/s^2 c- a car driving in a circle at a constant speed of 20 m/s d- a motorcycle with a constant velocity

The answer is D- a motorcycle with a constant velocity.

Velocity is much more stable in terms of speed and distance that can be associated with the dynamics of equilibrium (balance) compared to speed alone. It brings the constant relationship between the two elements. 

Speed is only focused on time of travel and not with direction. While acceleration does not show equilibrium but only a change in speed.

Is there a picture to go along with this