Four boxes are sliding with constant speed before each box experiences an unbalanced force of 8 N. Which box would experience the greatest acceleration when the unbalanced force is applied?

The object with the smallest mass.

Explanation:

According to Newton's 2nd law:

The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. In other words Acceleration=Force/mass; The smaller the mass, the greater the acceleration. The object with the smallest mass will have the largest acceleration.