Which of the following pairs of elements forms amphoteric oxides?

Analyze the question: An amphoteric oxide reacts with both acids and bases to form salts and water. We need the pair in which both elements form amphoteric oxides.

Concept: Amphoteric oxides are usually formed by metals or metalloids on the borderline between metallic and non-metallic character. Common examples: Al₂O₃, ZnO, PbO, SnO, BeO.

Checking each option:

• A. Be and Mg: BeO is amphoteric, but MgO is purely basic. ✗
• B. Na and K: Na₂O and K₂O are strongly basic. ✗
• C. B and Al: Al₂O₃ is the classic amphoteric oxide. B₂O₃ is acidic (it forms boric acid in water), though it shows some weakly amphoteric behaviour in some textbooks. Al₂O₃ is certain, and B is a borderline case.
• D. Si and Pb: PbO is amphoteric, but SiO₂ is acidic. ✗

Disagreement to flag: Strictly, only option C contains a definite amphoteric oxide with a reasonable claim for the second element. Option A (Be and Mg) and D (Si and Pb) each have one amphoteric oxide and one that is clearly not. In C, Al₂O₃ is certainly amphoteric, and B₂O₃ is generally classed as acidic but sometimes described as weakly amphoteric.

Answer: C

Tip: Memorize the amphoteric oxides with the mnemonic "Zn Al Pb Sn Be": ZnO, Al₂O₃, PbO, SnO, BeO. Then find the option where the elements are closest to this list. Here, Al in option C is the strongest match.