Explanation

(c) Let intensity of light coming from each slit of a coherent source is I.
As first slit has width 4 times the width of the second slit, so
$I_{1} = 4I\ and\ I_{2} = I$
$\therefore\ \frac{I_{\max}}{I_{\min}} = \frac{\left( \sqrt{I_{1}} + \sqrt{I_{2}} \right)^{2}}{\left( \sqrt{I_{1}} - \sqrt{I_{2}} \right)^{2}} = \frac{\left( \sqrt{4I} + \sqrt{I} \right)^{2}}{\left( \sqrt{4I} - \sqrt{I} \right)^{2}} = \frac{9}{1}$