Which one of the following acids is the weakest?

In the reaction $HNO_3 + H_2O \rightleftharpoons H_3O^{+} + NO_3^{-}$,the conjugate base of $HNO_3$ is = .......

Identify a species which is not a Bronsted acid but a Lewis acid.

Which of the following species is an acid and also a conjugate base of another acid?

Which of the following is the weakest base?

Phosphoric acid ionizes in three steps with their ionization constant values $K_{a_1}, K_{a_2}$ and $K_{a_3}$ respectively,while $K$ is the overall ionization constant. Which of the following statements are true?
$A.$ $\log K = \log K_{a_1} + \log K_{a_2} + \log K_{a_3}$
$B.$ $H_3PO_4$ is a stronger acid than $H_2PO_4^{-}$ and $HPO_4^{2-}$
$C.$ $K_{a_1} > K_{a_2} > K_{a_3}$
$D.$ $K_{a_1} = \frac{K_{a_3} + K_{a_2}}{2}$
Choose the correct answer from the options given below