The pH of 0.1M solution of the following compounds increases in the order

$HCl$ is strong acid, so that $HCl$ has the lowest pH. $NaCN$ is a salt of weak acid strong base so it has a highest pH value, whereas $NaCl$ is salt of strong acid and strong base. So, $NaCl$ has lower pH than $NaCN$. $N{H}_{4}Cl$ is a salt of weak acid and weak base, so it has a lower pH than both $NaCl$ and $NaCN$.

Conclusion Statement: Hence, the correct option is (2) $HCl$ < $N{H}_{4}Cl$< $NaCl$ < $NaCN$