Explanation

(b)
Elevation in boiling point is colligative property and depends upon number of ions of molecules or particles.
$CaSO_{4} \rightarrow Ca^{2 +} + SO_{4}^{2 -}$ $\ \ \ \therefore$ 2 ions
$BaCl_{2} \rightarrow Ba^{2 +} + 2Cl^{-}$ $\therefore$ 3 ions
NaCl $\rightarrow$ $Na^{+} + Cl^{-}$ $\therefore$ 2 ions
urea $\rightarrow$ no dissociation $\therefore$ 1 molecule
$\because BaCl_{2}$furnishes maximum ions.
$\therefore BaCl_{2}$ will have maximum boiling point.