H2A⇌HA−+H+HA−⇌H++A2−K1=[H2A][HA−][H+]…(1)K2=[HA−][H+][A2−]…(2) For the reaction $\begin{aligned}
& \mathrm{H}_2 \mathrm{A} \rightleftharpoons 2 \mathrm{H}^{+}+\mathrm{A}^{2-} \
& \mathrm{K}=\frac{\left[\mathrm{H}^{+}\right]^2\left[\mathrm{~A}^{2-}\right]}{\left[\mathrm{H}_2 \mathrm{A}\right]}=\mathrm{K}_1 \times \mathrm{K}_2 \
& =1 \times 10^{-5} \times 5 \times 10^{-10} \
& =5 \times 10^{-15}
\end{aligned}$ Hence, (C) is correct.