Final answer:
According to Newton's second law of motion, if more mass is added to the box while a constant force is applied, the acceleration of the box will decrease.
Step-by-step explanation:
If Jim applies a 15 N force to a box and then adds more mass to the box, according to Newton's second law of motion, the acceleration of the box will decrease. This is because the law states that acceleration is directly proportional to force and inversely proportional to mass. If the amount of force applied remains constant, and the mass increases, the acceleration will decrease. Therefore, the correct answer is A. The acceleration will decrease.