Explanation

(b) $\beta = \frac{(a + b)\lambda}{2a(\mu - 1)\alpha},\ i.e.,\ \beta \propto \frac{\lambda}{(\mu - 1)}$
When placed in water $\beta' \propto \frac{\frac{\lambda}{\mu^{'}}}{\left( \frac{\mu}{\mu^{'}} - 1 \right)}$
$i.e.,\ \beta' \propto \frac{\lambda}{(\mu - \mu')}$ but $< \mu$
$\therefore\frac{\beta^{'}}{\beta} = \frac{(\mu - 1)}{(\mu - \mu')}\ \ \because\mu^{'} > 1\lambda\ \therefore\beta^{'} > \beta$
$i.e.,$ the fringe width increases