Which one of the following is not a base?

Which of the following is the strongest conjugate base?

Aqueous ammonia is used instead of aqueous $NaOH$ solution to precipitate $Al^{3+}$ ions as $Al(OH)_3$ because .................

The pair of compounds which cannot exist together in solution is

Three reactions involving $H_2PO_4^-$ are given below :
$(i) \; H_3PO_4 + H_2O \rightarrow H_3O^{+} + H_2PO_4^-$
$(ii) \; H_2PO_4^- + H_2O \rightarrow HPO_4^{2-} + H_3O^{+}$
$(iii) \; H_2PO_4^- + OH^{-} \rightarrow H_2O + HPO_4^{2-}$
In which of the above does $H_2PO_4^-$ act as an acid?

Which of the following is not a Lewis acid?