Let $\beta$ be fringe width in air and $\beta'$ be fringe width in liquid.

Since $10th$ bright fringes in liquid coincide with 6 dark fringe in air.

$10\;\beta'=5.5 \beta$

Also, $ \large\frac{\beta}{\beta'}$$=\mu$

$\therefore \mu =\large\frac{10}{5.5}$

$\qquad= 1.8$

Hence b is the correct answer.