Explanation

(a)
Depression in freezing point is a colligative property. It depends on number of particles. More the number of particles, more will be depression in freezing point.
$K_{2}SO_{4} \rightarrow 2K^{+} + SO_{4}^{2 -}\ \ \ \$
It gives 3 particles.
$NaCl \rightarrow Na^{+} + Cl^{-}$
$\$It gives 2 particles.
Urea$\rightarrow$ No dissociation
Glucose $\rightarrow$ No dissociation.
$\therefore$ $K_{2}SO_{4}$ produces maximum number of particles
$\therefore$ $K_{2}SO_{4}$ has maximum depression in freezing point.