Which one of the following pair of solutions are expected to be isotonic at the same temperature?

Question

$\begin{array}{1 1}(a)\;\text{0.1M urea and 0.1M KCl}\\(b)\;\text{0.1M }Ca(NO_3)_2\;\text{and 0.1M}\;K_2SO_4 \\(c)\;\text{0.1M KCl and 0.1M }Na_2SO_4\\(d)\;\text{0.1M urea and 0.2M }MgCl_2\end{array}$