Find the number of aqueous solution which turn litmus paper blue.

(a) ${\mathrm{NH}}_4\mathrm{OH}$   (b) $\mathrm{NaOH}$

(c) ${\mathrm{NH}}_4\mathrm{Cl}$     (d) ${\mathrm{CH}}_3\mathrm{COONa}$

(e) ${\mathrm{NaHSO}}_4$   (f) $\mathrm{HCl}$

Water and ammonia react to generate ammonium hydroxide. Due to these hydroxide ions, red litmus turns blue.

Since $\mathrm{NaOH}$is a base and bases turn red litmus blue, red litmus paper will turn blue when exposed to a solution containing $\mathrm{NaOH}$.

because ${\mathrm{CH}}_3\mathrm{COONa}$is a salt of a strong base and a weak acid. As a result, it turns red litmus blue at higher temperatures and becomes more alkaline.

$\left(\mathrm{a}\right),\left(\mathrm{b}\right),\left(\mathrm{d}\right)$