An aqueous solution of a salt turns blue litmus to red. The salt could be the one obtained by the reaction of :

An aqueous solution of a salt that turns blue litmus to red must be acidic in nature. Acidic salts are formed by the neutralization reaction between a strong acid and a weak base.

• Option A ($\mathrm{HNO_3}$ and $\mathrm{NaOH}$): $\mathrm{HNO_3}$ is a strong acid and $\mathrm{NaOH}$ is a strong base. They react to form a neutral salt ($\mathrm{NaNO_3}$), which does not change the colour of litmus.
• Option B ($\mathrm{H_2SO_4}$ and $\mathrm{KOH}$): $\mathrm{H_2SO_4}$ is a strong acid and $\mathrm{KOH}$ is a strong base. They react to form a neutral salt ($\mathrm{K_2SO_4}$), which does not change the colour of litmus.
• Option C ($\mathrm{CH_3COOH}$ and $\mathrm{NaOH}$): $\mathrm{CH_3COOH}$ is a weak acid and $\mathrm{NaOH}$ is a strong base. They react to form a basic salt ($\mathrm{CH_3COONa}$), which turns red litmus to blue.
• Option D ($\mathrm{HCl}$ and $\mathrm{NH_4OH}$): $\mathrm{HCl}$ is a strong acid and $\mathrm{NH_4OH}$ is a weak base. They react to form an acidic salt ($\mathrm{NH_4Cl}$). In water, this salt hydrolyses to produce an acidic solution containing excess hydronium ions, which turns blue litmus paper red.