Explanation

(b) Fringe visibility
$V = \frac{2\sqrt{I_{1}I_{2}}}{I_{1} + I_{2}}$
Where $I_{1}$ and $I_{2}$ are intensities of coherent sources.
Given, $\frac{I_{1}}{I_{2}} = \frac{1}{4}$
$\therefore I_{2} = 4I_{1}$
$\therefore Fringe\ visibility = \frac{2\sqrt{I_{1} \times 4I_{1}}}{\left( I_{1} + 4I_{1} \right)}$
$= \frac{2 \times 2I_{1}}{5I_{1}} = \frac{4}{5}$
$\Rightarrow V = 0.8$