Final answer:
At the equilibrium position of a pendulum, velocity is at a maximum.
Step-by-step explanation:
At the equilibrium position of a pendulum, the quantity at a maximum is velocity. The equilibrium position is the point where the pendulum would naturally come to rest if there were no outside forces acting upon it, and at this point, the pendulum has zero displacement. Because all of the potential energy is converted into kinetic energy, the pendulum has its highest velocity as it passes through equilibrium. Hence, the correct answer to the question is D. Velocity.