What is a Lewis acid?

Which of the following statements is true?

Considering the following reactions,which pair of reactions shows that water has basic properties?
$(i) \ CO_2 + H_2O \rightleftharpoons H_2CO_3$
$(ii) \ NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$
$(iii) \ HCl + H_2O \rightleftharpoons Cl^- + H_3O^+$

Conjugate acid of $NH_2^{-}$ and $NH_3$ are respectively

Complete the following Acid-Base reactions and identify each reactant as an Acid or a Base:
$(i)$ $CH_3NH_2 + H_2O$
$(ii)$ $CO_2 + H_2O$
$(iii)$ $H_2PO_4^- + CO_3^{2-}$
$(iv)$ $NH_2NH_2 + H_2O$
$(v)$ $C_6H_6 + NO_2^+$
$(vi)$ $C_6H_6 + NH_2^-$

Which of the following is the strongest conjugate base?