Explanation

(b)
Osmotic pressure $\propto$ number of particles.
$\therefore$ Solution with least number of particles will have minimum osmotic pressure.
(i) NaCl $\rightarrow \ \ \ Na^{+}\ + Cl^{-}\ \ \ (2\ ions)\ \$
$\therefore$ Concentration of particles in
NaCl=2$\times$2M =4M
(ii) Glucose does not dissociate
$\therefore$ Concentration of particles
= 1$\times$1M= 1M
Urea does not dissociate
$\therefore$ Concentration of particles 1$\times$2M=2M
$\therefore$ Glucose solution will have minimum osmotic pressure.