Which of the following acid is stronger than benzoic acid $\left({\mathrm{K}}_{\mathrm{a}}=6.3\times {10}^{-5}\right)?$

$$\begin{gathered} \because {\mathrm{pK}}_{\mathrm{a}}=-\log {\mathrm{K}}_{\mathrm{a}} \\ \therefore {\mathrm{pK}}_{\mathrm{a}}\text{ for benzoic acid }=-\log \left(6.3\times {10}^{-5}\right) \\ =-\log 6.3-\log {10}^{-5} \\ =-0.799+5=4.201 \\ \text{ (a) }{\mathrm{pK}}_{\mathrm{a}}\text{ for }\mathrm{A}=-\log \left(1.67\times {10}^{-8}\right) \\ =-\log 1.67-\log {10}^{-8} \\ =-0.222+8=7.778 \\ \text{ (b) }{\mathrm{pK}}_{\mathrm{a}}\text{ for }\mathrm{B}=6\text{, } \\ \text{ (c) }{\mathrm{pK}}_{\mathrm{a}}\text{ for }\mathrm{C}=4.0 \\ \text{ (d) }{\mathrm{pK}}_{\mathrm{a}}\text{ for }\mathrm{D}=-\log \left(1.0\times {10}^{-5}\right) \\ =-\log 1.0-\log {10}^{-5}=0+5=5 \end{gathered}$$
For a stronger acid, pK_a must be less. Therefore, acid C will be stronger than benzoic acid.