Which one is not an acid salt?

$pK_a$ values of two acids $A$ and $B$ are $4$ and $5$. The strengths of these two acids are related as:

Which of the following species can act as both a $Br\text{ø}nsted$ acid and a $Br\text{ø}nsted$ base?

$HSO_4^-$ is the conjugate base of

Which of the following is the weakest acid?

Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R:$
Assertion $A :$ The amphoteric nature of water is explained by using Lewis acid/base concept.
Reason $R :$ Water acts as an acid with $NH_{3}$ and as a base with $H_{2}S$.
In the light of the above statements,choose the correct answer from the options given below: