An ac source is connected to a capacitor. The current in the circuit is I. Now, a dielectric slab is inserted into the capacitor, then the new current is :

On inserting a dielectric slab into a capacitor its capacitance will increase which reduces its capacitive reactance, there by the current in the circuit for a given AC source would increase. As the current in the circuit is I, on inserting a dielectric slab current will be more than I.