Variable oxidation states indicate transition metals

Steps:

• Identify common oxidation states: +2 and +3 suggest variable valency beyond s-block.
• Recall s-block groups: Group II (alkaline earths) fixed at +2; Group III (boron family) primarily +3.
• Note p-block: Group V (nitrogen family) shows +5 or -3, not matching +2/+3 pair.
• Conclude d-block: Transition metals display multiple states like +2/+3 due to partial d-orbital filling.

Why D is correct:

• Transition metals exhibit variable oxidation states from 0 to +8, including +2 and +3, as d-electrons participate in bonding per the general formula for their compounds.

Why the others are wrong:

• A: Group II elements form only +2 ions, losing two s-electrons.
• B: Group III elements predominantly show +3 oxidation state by losing three valence electrons.
• C: Group V elements typically display +5 (for metals) or -3 (for non-metals), not +2/+3.

Final answer: D