The correct statement is:
The increase in the energy stored in the system is proportional to the decrease in kinetic energy.
When the second bar magnet spins and attaches to the first magnet, its kinetic energy decreases as it becomes more stable and static. The energy that was in the form of kinetic energy is transformed into potential energy in the magnetic field as the magnets align and attract each other. Thus, the increase in energy stored in the magnetic field is associated with a decrease in kinetic energy.