A solenoid with an iron core and a bulb in series are connected to a d.c. source. When the iron core is removed from the solenoid, the brightness of the bulb will

cease to zero

The correct answer is Option (4) → cease to zero

When a solenoid with an iron core and a bulb in series is connected to a d.c. source, initially the current builds up gradually due to the inductance of the solenoid. After steady state, a d.c. current flows. However, if the bulb is in series and the solenoid is ideal (no resistance), removing the iron core drastically reduces the induced EMF that was helping current flow through the bulb. In such a hypothetical ideal case, the current can drop to zero.

Brightness of the bulb = 0 (ceases to zero)