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

(i) HCl is a strong acid. Hence its pH is lowest among the others. 

(ii) NaCl is a salt of strong acid and strong base so it is not hydrolyzed and hence its pH is 7.

(iii) ${\mathrm{NH}}_4\mathrm{Cl}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{NH}}_4\mathrm{OH}+\mathrm{HCl}$

$\therefore$ The solution is acidic and pH is less than that of 0.1 M HCl. 

(iv) $\mathrm{NaCN} + {\mathrm{H}}_2\mathrm{O} \rightleftharpoons \mathrm{NaOH} + \mathrm{HCN}$

$\therefore$ The solution is basic and pH is more than that of 0.1 M HCl. 

$\therefore$Correct order for increase in pH is

$\mathrm{HCl} < {\mathrm{NH}}_4\mathrm{Cl} < \mathrm{NaCl} < \mathrm{NaCN}$.