Which of the following solutions will freeze at the lowest temperature?

$\begin{array}{1 1}(a)\;\text{2% urea solution}\\(b)\;\text{4% urea solution}\\(c)\;\text{6% urea solution}\\(d)\;\text{8% urea solution}\end{array}$

Depression of Freezing point, $\Delta\; T_f = i\;K_f\; m$, where $i$ is the van't Hoff factor, $K_f$ is the Freezing Point Depression Constant and $m$ is the molality.
In this case $i$ and $K_f$ is the same across all solutions.
Molality $m$ is the number of moles of solute per Kg of solvent. Hence larger the % of the solute, higher the molarity and higher the freezing point.
Therefore, the 8% urea solution will have high molality and hence higher freezing point.
edited Jul 16, 2014