a b c d

c

$ \text{Intensity is propotional to slit width}$ $ I_1 : I_2 = 1:9 $ $\frac{I_{max}}{I_{min}} = \frac{(\sqrt I_1 - \sqrt I_2)^2}{(\sqrt I_1 + \sqrt I_2)^2} = \frac{(3 - 1)^2}{(3 + 1)^2} = \frac{1}{4}$