Suppose the net forceon an object is a nonzero constant.Which of the following could also be constant? (A) Position.(B) Speed. (C) Velocity.(D) Acceleration.

The correct option is: (D) Acceleration.
From Newton’s second law of motion, one has


If the net force on an object is a nonzero constant, the acceleration, which is in direction of net force, would neither change its direction or its magnitude. This ensures that the magnitude of the net force and its direction remains constant throughout.
If the acceleration of the object has a constant direction and magnitude, the acceleration vector would also remain constant.
Therefore (D) is the correct option.
If the object has constant acceleration, the change in velocity vector of the object must also remain constant. To ensure this, the velocity of the object must change linearly over time.
Therefore, both speed and velocity will not be constant ruling out options (B) and (C).
Similarly, the velocity of the object at any instant is the rate of change of its position. From above it is clear that the velocity of the object must change to ensure that acceleration of the object is constant and is in the direction of force.
Therefore, the position of the particle must itself be a function of time and cannot remain constant thereby ruling option (A).