What happens if the applied voltage is below the seal-in voltage?

When the applied voltage is below the seal-in voltage, the relay is unable to maintain its activated state. The seal-in voltage is the minimum voltage required to keep the relay energized after it has been initially activated. If the voltage drops below this threshold, the magnetic field that holds the armature in its position weakens, leading to the armature releasing. This action essentially deactivates the relay, causing it to return to its default, unenergized position.

In contrast, the other options describe outcomes that do not occur under these circumstances: the relay will not activate if the voltage is insufficient, the core remains in place when the armature releases, and overheating of the coil is not a direct consequence of a drop below the seal-in voltage but rather relates to excessive voltage or current levels.