Final answer:
To restore a solenoid to its de-energized state, the electrical current passing through it needs to be removed.
Step-by-step explanation:
When a solenoid is de-energized, it is restored to its de-energized state by removing the electrical current passing through it. This can be done by opening the circuit or disconnecting the power source. Once the current is removed, the magnetic field generated by the solenoid collapses, and it returns to its original state.