Consider the following salts. Which one(s) when dissolved in water will produce a acid solution?
$1.RbCl{O}_{4}2.NaN{O}_{3}3.N{H}_{4}Cl4.NaCl$

Salt of strong acid and weak base produces acidic solution 

So,  ${NH}_{4}Cl\Rightarrow \left({NH}_{4}OH+\underbrace{HCl}\right)$

                                                    $S.A$

others are made from $(S.A+S.B)$ $\Rightarrow$ Neutral in $H_{2}O$

Thus, only $N{H}_{4}Cl$ will produce acid solution.