In a closed system, the net force is given by Newton's second law, which states that \( F = m \cdot a \) (force equals mass times acceleration). To keep the net force constant while changing the system, you can adjust the mass and acceleration in such a way that their product remains the same.
The option "increase the mass and decrease the acceleration by the same numeric values" is a correct approach. If you increase the mass while simultaneously decreasing the acceleration in such a way that the product \( m \cdot a \) (the net force) remains constant, you effectively change the system without altering the net force.
Thus, the correct answer is:
increase the mass and decrease the acceleration by the same numeric values.