The $H^{+}$ ion is a ...... .

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:

For the reaction $NH_4^+ + S^{2-} \rightleftharpoons NH_3 + HS^-$,identify the nature of $NH_3$ and $HS^-$ in the context of the Brønsted-Lowry theory.

In which of the following reactions is the backward reaction favoured?

Derive the equation of the relation between the weak base ionization constant $K_b$ and its conjugate acid ionization constant $K_a$.

Identify the amphoteric species among the following:
$(I) \, H_2O$
$(II) \, NH_2^-$
$(III) \, H_2PO_4^-$
$(IV) \, HCO_3^-$