b
(b) Let the intensity of the two waves be $I_{1}$ and $I_{2}$
Given$:$ $I_{2}: I_{1}=9: 1 \Longrightarrow I_{2}=9 I_{1}$
Now $\frac{I_{\max }}{I_{\min }}=\frac{(\sqrt{I_{1}}+\sqrt{I_{2}})^{2}}{(\sqrt{I_{2}}-\sqrt{I_{1}})^{2}}=\frac{(\sqrt{I_{1}}+\sqrt{9 I_{1}})^{2}}{(\sqrt{9 I_{1}}-\sqrt{I_{1}})^{2}}=\frac{16 I_{1}}{4 I_{1}}$
$\Longrightarrow I_{\max }: I_{\min }=4: 1$