Which one of the following is an isotonic pair of solutions?

Question

$$\begin {array} {1 1}(a)\;0.15\;M\;NaCl\;and\;0.1\;M\;Na_2SO_4 \\ (b)\;0.2\;M\;urea\;and\;0.1\;M\;sugar \\ (c)\;0.1\;MM\;BaCl_2\;and\;0.1\;M\;urea \\ (d)\;0.2\;M\;MgSO_4\;and\;0.1\;M\;NH_4Cl\end {array}$$