If a compound AB dissociates to the extent of 75 % in an aqueous...

$\alpha = 0 . 75, n = 2$

$i = 1 - \alpha + n\alpha = 1 - 0 . 75 + 2 \times 0 . 75 = 1 . 75$

$\Delta{T}_{b} = {ik}_{b}m$

or $2 . 5 = 1 . 75 \times 0 . 52 \times m$

or, $m = \dfrac{2 . 5}{1 . 75 \times 0 . 52} = 2 . 74$

$∴$ nearest integer answer will be $3$