Which one of the following mixtures creates a buffer solution?

For the given case, $(\mathrm{N}\times \mathrm{V})$ of $\mathrm{NaOH}$ is greater than the $(\mathrm{N}\times \mathrm{V})$ of $\mathrm{HF}$ for buffer solution formation.

Thus, $15.0 \mathrm{mL} 0.10 \mathrm{M} \mathrm{NaOH}+20.0 \mathrm{mL} 0.10 \mathrm{M} \mathrm{HF}$ mixture will provide the buffer solution.