Which of the following aqueous salt solutions is acidic?

Sodium chloride is a salt of a strong acid ($\text{HCl}$) and a strong base ($\text{NaOH}$), so its aqueous solution is neutral.

Ammonium nitrate is a salt of a strong acid ($\text{HNO}_3$) and a weak base ($\text{NH}_4\text{OH}$), so its aqueous solution is acidic due to the cationic hydrolysis of the ammonium ion.

Potassium cyanide is a salt of a weak acid ($\text{HCN}$) and a strong base ($\text{KOH}$), so its aqueous solution is basic.

Sodium carbonate is a salt of a weak acid ($\text{H}_2\text{CO}_3$) and a strong base ($\text{NaOH}$), so its aqueous solution is basic.

Answer: Ammonium nitrate