In an interference pattern by two identical slits, intensity of central maxima is I. If one slit is closed, intensity of central maxima changes to $I_0$. Then I and $I_0$ are related by

$I = I_0$ $I = 2I_0$ $I = 3I_0$ $I = 4I_0$

$I = 4I_0$

$I_1 = I_2 = a^2$ $I_{max.} = (a+a)^2 = 4a^2 = I$ If one slit is closed, Intensity, $I_0 = (a)^2$ $∴\frac{I}{I_0}=\frac{4a^2}{a^2}=4$ or $I = 4I_0$