What is the conjugate acid of $HPO_4^{2-}$?

Which are the conjugate bases of the strong acids $HClO_4$,$H_2SO_4$,$HNO_3$,and $HPO_4^{2-}$? Are they strong or weak?

In the following reaction:
$[Al(H_2O)_6]^{+3} + HCO_3^{-} \rightleftarrows [Al(H_2O)_5OH]^{+2} + H_2CO_3$
$(A)\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad(B)\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad(C)\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad(D)$
Which species behave as Bronsted-Lowry acids?

Acidity of $BF_3$ can be explained on which of the following concepts?

What is a Lewis acid?

Which of the following is the weakest acid?