A solution of 10 g of non-volatile binary electrolyte (mol wt=100) in 500 g of water freezes at $-0.74^{\circ}C$. What is the degree of ionisation ? ($k_f$ of water$=1.85\;K \; molality^{-1})$

Question

$$\begin {array} {1 1} (a)\;50\% & \quad (b)\;75\% \\ (c)\;100\% & \quad (d)\;0\% \end {array}$$