Dear Ankita
We know that the capacitance is directly proportional to the amount of change in the capacitor at a constant voltage. Now since the charge doubled keeping the voltage constant that means the capacitance of the capacitor is doubled after inserting the dielectric material. Now again we know that the capacitance is directly proportional the dielectric constant. Nos since the dielectric constant of the air is almost 1 so the dielectric constant of the material is 2.
Regards
Arun (askIITians forum expert)
