Which pair of solutes could be used to prepare an aqueous buffer solution with a $\mathrm{pH} < 7$?

Although (b) and (c) both can form buffer at $\mathrm{pH}<7$, but $\mathrm{HF}-\mathrm{NaF}$ being an acidic buffer has more tendency to form buffer at a $\mathrm{pH} < 7$.

Correct option is B.