Which of the following represent the correct order of increase in covalent character?

Question

$\begin{array}{1 1}(a)\;NaC l < MgCl_2 < AlCl_3\\(b)\; AlC l_3 < MgCl_2 < NaCl\\(c) \;AlC l_3 < NaCl < MgCl_2\\(d)\; NaC l < AlCl_3 < MgCl_2\end{array}$