Explanation

(b)
Depression in freezing point is colligative property. The solute which produces highest number of ions will have minimum freezing point .
One molal NaCl aqueous solution
$NaCl\ \ \ \ \ \rightarrow Na^{+} + Cl^{-}$
$\therefore$ 2 ions/molecule
One molal $CaCl_{2}$ solution
$CaCl_{2}\ \ \ \ \rightarrow \ Ca^{2 +} + 2Cl^{-}$
$\ \ \ \ \ \ \ \ \therefore$ 3 ions/molecule
One molal KCl aqueous solution
KCl $\rightarrow$ $K^{+} + Cl^{-}$
$\therefore$ 2 ions/molecule
One molal urea aqueous solution $\rightarrow$no dissociation
$\because\ \ \$CaC$l_{2}$ solution has highest number of ions
$\therefore$ It has lowest freezing point.