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

Neutralization of an acid with a base invariably results in the production of

The values of dissociation constants $(K_a)$ of some acids (at $25\,^{\circ}C$) are as follows. Indicate which is the strongest acid in water:
$(A)$ $1.4 \times 10^{-2}$
$(B)$ $1.6 \times 10^{-4}$
$(C)$ $4.4 \times 10^{-10}$
$(D)$ $4.3 \times 10^{-7}$

The strongest Bronsted base among the following anions is

On dissolving $1 \ mole$ of each of the following acids in $1 \ L$ of water,the acid which does not give a solution of strength $1 \ N$ is:

The dissociation constants of two acids $HY$ and $HZ$ are $2.88 \times 10^{-4}$ and $1.8 \times 10^{-5}$ respectively. The relative strength of the acids will be :-