c
\(HA \leftrightarrow {H^ + } + {A^ - }\)

(Unionized, weak acid and common ion effect)

\(HA + NaOH \to NaA + {H_2}O\)

\(NaA \to N{a^ + } + {A^ - }\)      (ionized)

\({K_a} = \frac{{[{H^ + }][{A^ - }]}}{{[HA]}}\)

Given,  \(pH = 6,\,[{H^ + }] = 1 \times {10^{ - 6}}\)

\([{H^ + }] = \frac{{{K_a}[Acid]}}{{[Salt]}}\)

\(\frac{{[Salt]}}{{[Acid]}} = \frac{{{K_a}}}{{[{H^ + }]}} = \frac{{{{10}^{ - 5}}}}{{{{10}^{ - 6}}}} = 10:1\)