In the following sets of reactants which two sets best exhibit the...

For a substance to be called amphoteric, it should be able to react as a base as well as an acid. Hence, when it is reacted with a base, it should act as an acid, and when it is reacted with an acid, it should behave as a base.

The reactions for amphoteric nature of Al₂O₃. x H₂O(s) are

${\text{Al}}_{2} {\text{O}}_{3} . x {\text{H}}_{2} \text{O} \left(\text{s}\right) \overset{{\text{OH}}_{\text{aq}}^{-}}{\to} \text{Al} {\left(\text{OH}\right)}_{4}^{-} aq$
Aluminate anion

${\text{Al}}_{2} {\text{O}}_{3} . x {\text{H}}_{2} \text{O} \left(\text{s}\right) \overset{{\text{H}}_{\text{aq}}^{+}}{\to} {\left[\text{Al} {\left({\text{H}}_{2} \text{O}\right)}_{6}\right]}_{\text{aq}}^{3 +}$.

[Al(H₂O)₆]³⁺ is octahedral complex ion

[Al(OH)₄]^-1 tetrahedral at pH = 13