Identification of Sets Containing Exclusively Amphoteric Oxides

An amphoteric oxide is an oxide that can react with both acids and bases to produce salt and water, displaying both acidic and basic properties.

Let us evaluate the nature of each oxide present in the options:

1. Option (A):

   • $\text{Cr}_2\text{O}_3$ (Chromium in $+3$ oxidation state): It is amphoteric; it reacts with acids to form $\text{Cr}^{3+}$ ions and with concentrated bases to form chromite ions, $[\text{Cr}(\text{OH})_4]^-$.
   • $\text{BeO}$: It is amphoteric; it reacts with acids to form $\text{Be}^{2+}$ ions and with bases to form beryllate ions, $[\text{Be}(\text{OH})_4]^{2-}$.
   • $\text{SnO}$: It is amphoteric; it dissolves in acids to give $\text{Sn}^{2+}$ ions and in alkalis to form stannite ions, $[\text{Sn}(\text{OH})_3]^-$.
   • $\text{SnO}_2$: It is amphoteric; it reacts with acids to give $\text{Sn}^{4+}$ ions and with alkalis to form stannate ions, $[\text{Sn}(\text{OH})_6]^{2-}$.

   Since all oxides ($\text{Cr}_2\text{O}_3$, $\text{BeO}$, $\text{SnO}$, $\text{SnO}_2$) in Option (A) are amphoteric, Option (A) is correct.

2. Option (B):

   • $\text{CrO}$ (Chromium in $+2$ oxidation state): Oxides of metals in lower oxidation states are generally basic. Thus, $\text{CrO}$ is a basic oxide.

   Since $\text{CrO}$ is basic, Option (B) does not contain exclusively amphoteric oxides. Thus, Option (B) is incorrect.

3. Option (C):

   • $\text{NO}$: It is a neutral oxide.
   • $\text{B}_2\text{O}_3$: It is an acidic oxide as it reacts with water to give boric acid, $\text{B}(\text{OH})_3$.

   Since $\text{NO}$ and $\text{B}_2\text{O}_3$ are not amphoteric, Option (C) is incorrect.

4. Option (D):

   • $\text{ZnO}$: It is amphoteric; it reacts with acids to form $\text{Zn}^{2+}$ and with strong alkalis to form zincate ions, $[\text{Zn}(\text{OH})_4]^{2-}$.
   • $\text{Al}_2\text{O}_3$: It is amphoteric; it reacts with acids to form $\text{Al}^{3+}$ and with strong bases to form aluminate ions, $[\text{Al}(\text{OH})_4]^-$.
   • $\text{PbO}$: It is amphoteric; it reacts with acids to form $\text{Pb}^{2+}$ and with strong bases to form plumbite ions, $[\text{Pb}(\text{OH})_3]^-$.
   • $\text{PbO}_2$: It is amphoteric; it reacts with acids and with bases to form plumbate ions, $[\text{Pb}(\text{OH})_6]^{2-}$.

   Since all oxides ($\text{ZnO}$, $\text{Al}_2\text{O}_3$, $\text{PbO}$, $\text{PbO}_2$) in Option (D) are amphoteric, Option (D) is correct.

Conclusion: The correct choices containing exclusively amphoteric oxides are A and D.