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

When $HClO_4$ is dissolved in the weak acid $HF$,the following equilibrium is established:
$HF + HClO_4 \rightleftharpoons H_2F^+ + ClO_4^-$
Which of the following is the correct set of conjugate acid-base pairs?

Which of the following is a Lewis acid but not a Bronsted acid?

Which of the following is the strongest Bronsted base?

In which of the following reactions does $NH_3$ act as an acid?

Identify base $2$ for the following equation according to the $BRONSTED-LOWRY$ theory.
$HCl_{(aq)} + H_2O_{(l)} \rightleftharpoons H_3O_{(aq)}^{+} + Cl_{(aq)}^{-}$