Question

The image shows one complete cycle of a mass on a spring in simple harmonic motion.

An illustration of a mass on a vertical spring with a transverse wave showing the position of the mass on the spring will make a transverse wave shape if bouncing up and down and moving horizontally. The shortest spring has the mass at the top of each crest and are labeled A and E respectively. The position where the mass in in the trough of the transverse wave would be labeled C and stretches the spring the farthest. The middle length springs has masses where the equilibrium of the material of the medium containing the transverse wave would be at the equilibrium of the material would be and are labeled B and D.
Which describes the system at point D?

The velocity has the maximum upward value because the acceleration upward is at a maximum.
The net force is at a maximum in the downward direction, so the velocity begins to decrease.
The velocity is zero because the net force is zero.
The net force is zero, so the acceleration is zero.

Answer 1

"The net force is zero, so the acceleration is zero."

At the equilibrium (middle) position the spring force exactly balances gravity, so the net force (and therefore acceleration) is zero. (The speed is actually maximum there.)