Which among the following has highest $$\mathrm{pH}$$ ?

$$K_a$$ values for acids $$\mathrm{H}_2 \mathrm{SO}_3, \mathrm{HNO}_2, \mathrm{CH}_3 \mathrm{COOH}$$ and $$\mathrm{HCN}$$ are respectively $$13 \times 10^{-2}, 4 \times 10^{-4}, 1.8 \times 10^{-5}$$ and $$4 \times 10^{-10}$$, which of the above acids produces stronger conjugate base in aqueous solution?

The conjugate base of $$\mathrm{NH}_3$$ is

In the reaction

$$\mathrm{B}(\mathrm{OH})_3+2 \mathrm{H}_2 \mathrm{O} \rightarrow\left[B(\mathrm{OH})_4\right]^{-}+\mathrm{H}_2 \stackrel{+}{\mathrm{O}}, \mathrm{B}(\mathrm{OH})_3$$

functions as