Which one of the following solutions has a lowest freezing point?

Question

$\begin {array} {1 1} (1)\;0.1\: M\: Urea & \quad (2)\;0.1\: M\: Ba\: CI_2 \\ (3)\;0.2\: M\: Na\: NO_3 & \quad (4)\;0.2\: M\: Na_2\: SO_4 \end {array}$