Which of the following substances is the strongest acid and acts as a base in the reaction?

According to the Bronsted-Lowry concept,an acid is a substance which:

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 :-

Water is a/an.....

The conjugate base of $HPO_4^{2-}$ is:

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