Which among the following salts turns red litmus blue in its aqueous solution?

The salt that turns red litmus blue must produce a basic aqueous solution, which occurs when hydrolysis yields hydroxide ions. This requires a salt derived from a strong base and a weak acid.

For potassium cyanide (KCN), formed from strong base KOH and weak acid HCN, the cyanide ion hydrolyzes: $\text{CN}^- + \text{H}_2\text{O} \rightleftharpoons \text{HCN} + \text{OH}^-$, producing basic solution.

In contrast, NaNO$_3$, NaCl, and KCl derive from strong acids and strong bases, undergo no significant hydrolysis, and yield neutral solutions.

Thus, only KCN turns red litmus blue.