A bulb and a capacitor are connected in series to an a.c. source. A dielectric slab is now introduced between the plates of the capacitor. The intensity of the bulb will be:

Increases

When dielectric slab is inserted between plates of a capacitor the capacitance increases and hence its capacitive reactance $X_C =\frac{1}{\omega C}$ decreases and hence Impedance decreases. 

$ I = \frac{V}{Z}$ Increases.