When the second bar magnet spins and then attaches to the first magnet, there is an interaction between the two magnetic fields. Initially, the system has some amount of mechanical energy in the form of kinetic energy as the second magnet spins. When it attaches, some of this kinetic energy is transformed and the magnetic potential energy may change based on the configuration of the magnetic fields.
In this scenario, the key point is that energy is conserved. As the second magnet moves and then bonds to the first one, the kinetic energy decreases while the magnetic potential energy increases (assuming the magnets attract each other).
Therefore, the correct statement is:
The decrease in the energy stored in the system is proportional to the increase in kinetic energy.
In this case, since the magnets interact and attract, the kinetic energy of the spinning magnet decreases as it stops spinning and attaches to the stationary magnet, suggesting an increase in the energy stored in the magnetic field.