(i) We know that if the number of turns in the inductor
decreases, then inductance L decreases. So, the net resistance of the circuit decreases and hence, the current through the circuit increases, increasing the brightness of the bulb.
(ii) If the soft iron rod is inserted in the inductor, then the inductance L increases. Therefore, the current through the bulb will decrease, decreasing the brightness of the bulb.
(iii) If the capacitor of reactance Xc = XL is connected in series with the circuit, then
This is a case of resonance. In this case, the maximum current will flow through the circuit. Hence, the brightness of the bulb will increase.