An object is moving at a constant velocity. all but one of the following statements could be true. which one cannot be true?
a. two forces act simultaneously on the object.
b. a single force acts on the object.
c. three forces act simultaneously on the object.
d. no forces act on the object.

b.A single force acts on the object.

Explanation:

Newton's first law establishes that an object at rest remains at rest and an object in motion will have constant velocity if the result of the forces acting on them is zero

An object acceleration can be defined as the variation of velocity in an interval of time:                            

  (1)  

Where is the final velocity, is the initial velocity,   is the final time and is the initial time.

If the velocity is constant there is no variation of velocity, therefore, the acceleration will be 0.

Newton's second law can be used To show that no forces act on the object:

(2)

Where R is the resultant of all the forces, m is the mass and a is the acceleration.

Since there is no acceleration for an object with constant velocity, equation (2) can be expressed as:

So it is correct to say that an object with constant velocity has no forces acting on it.

In the other hand in the case of several forces acting on the object, The net force (R) can be zero if they have the same magnitude and opposite direction

So (a), (c) and (d) are correct because of what was previously discussed.

Note that b cannot be true since it violates Newton's first law, since the object is moving at a constant velocity.

im sorry i dont know