# Most likely the following mixtures to be an ideal solution is

$\begin{array}{1 1}(a)\;C_6H_5OH-H_2O\\(b)\;C_6H_6(l)-H_2O\\(c)\;NaCl-H_2O\\(d)\;C_2H_5OH-C_6H_6\end{array}$

$NaCl-H_2O$ system obeys Raoult's law,where as other mixture does not obey Raoult's law.
Hence (c) is the correct answer.