Two parallel plate capacitors, X and Y, have the same area of plates and the same separation between them. They are connected in series across a 12 V battery. X has air between the plates while Y contains a dielectric medium of dielectric constant 4. The ratio of electrostatic energy stored in X and Y is

4 : 1

The correct answer is Option (1) → 4 : 1

$C_X = C,\quad C_Y = 4C$

$\text{In series: charge } Q \text{ same}$

$U = \frac{Q^2}{2C}$

$\frac{U_X}{U_Y} = \frac{\frac{Q^2}{2C}}{\frac{Q^2}{2(4C)}}$

$\frac{U_X}{U_Y} = \frac{1}{1/4} = 4$

The ratio $U_X : U_Y = 4 : 1$.