Which of the following $0.1 \, M$ aqueous solutions has the lowest freezing point?

When mercuric iodide $(HgI_2)$ is added to an aqueous solution of potassium iodide $(KI)$,the

Molal depression constant for a solvent is $4.0 \, K \, kg \, mol^{-1}$. The depression in the freezing point of the solvent for $0.03 \, mol \, kg^{-1}$ solution of $K_2SO_4$ is .............. $K$ (Assume complete dissociation of the electrolyte).

$80$ mole percent of $MgCl_2$ is dissociated in aqueous solution. The vapour pressure of $1.0$ molal aqueous solution of $MgCl_2$ at $38^{\circ} C$ is $.........$ $mm$ $Hg$. (Nearest integer)
Given : Vapour pressure of water at $38^{\circ} C$ is $50$ $mm$ $Hg$.

The osmotic pressure of a dilute solution of an ionic compound $XY$ in water is four times that of a solution of $0.01\, M\, BaCl_2$ in water. Assuming complete dissociation of the given ionic compounds in water,the concentration of $XY$ (in $mol\, L^{-1}$) in solution is

The Van't Hoff factor of $Ca(NO_3)_2$ is