At the same concentration, which acid would have the lowest pH?

$\begin{array}{*{20}{l}} {{\text{A.}}}&{{\text{HN}}{{\text{O}}_{\text{2}}}}&{{K_{\text{a}}} = 5.6 \times {{10}^{ - 4}}{\text{ mol}}\,{\text{d}}{{\text{m}}^{ - 3}}} \\ {{\text{B.}}}&{{\text{HF}}}&{{K_{\text{a}}} = 6.8 \times {{10}^{ - 4}}{\text{ mol}}\,{\text{d}}{{\text{m}}^{ - 3}}} \\ {{\text{C.}}}&{{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{COOH}}}&{{K_{\text{a}}} = 6.3 \times {{10}^{ - 5}}{\text{ mol}}\,{\text{d}}{{\text{m}}^{ - 3}}} \\ {{\text{D.}}}&{{\text{HCN}}}&{{K_{\text{a}}} = 4.9 \times {{10}^{ - 10}}{\text{ mol}}\,{\text{d}}{{\text{m}}^{ - 3}}} \end{array}$

B