Under what condition(s) will an object be in equilibrium? notice that equilibrium in physics has a broader definition.

(a) only if it is moving with constant velocity.
(b) only if it is at rest.
(c) only if it is moving with non-zero constant acceleration.
(d) if it is either at rest or moving with constant velocity.
(e) if it is either moving with constant velocity or with non-zero constant acceleration.

D. If it is either at rest or moving with constant velocity.

Explanation:

In physics, an equilibrium condition is one in which a system tends not to change its state of motion or its energy with time.

This definition has broad implications in various areas of physics, but when we are considering the motion of a body, we find that the since a body tends to resist change in motion (or more precisely, it resists acceleration ), the equilibrium condition is when the body is either at rest or moving with constant velocity.

If you disturb this equilibrium condition by applying a force, the body will accelerate for as long as the force is being applied; once this force is removed, the body will resume its constant velocity motion—but with greater velocity.

So, because the body tends to move at a constant velocity, and returns to moving at a constant velocity when all forces acting on it have been removed, we say that constant velocity motion is an equilibrium state.

The same is true for a body at rest, because a body at rest in your frame is always moving at constant velocity in another frame of reference.