Assertion (A) Covalent character follows $\mathrm{NaCl} < \mathrm{MgCl}_2 < \mathrm{BeCl}_2 < \mathrm{LiCl}$.
Reason (R) According to Fajan's rules, covalent character of ionic compounds increases with decrease in size of cation.
Both $A$ and $R$ are true and $R$ is a correct explanation for $A$.
Both $A$ and $R$ are true but $R$ is not a correct explanation for $\mathrm{A}$.
$A$ is true but $R$ is false.
$A$ is false but $R$ is true.
Solution
The correct order of covalent character is $\mathrm{NaCl} < \mathrm{LiCl} < \mathrm{MgCl}_2 < \mathrm{BeCl}_2$. The covalent character depends on small size as well as more + ve charge on cation.