Oxidation state increase identifies the oxidized species

Steps:

• Assign oxidation states: In SnCl₂, Sn is +2; in SnCl₄, Sn is +4. In HgCl₂, Hg is +2; in Hg₂Cl₂, each Hg is +1. Cl is -1 throughout.
• Compare changes: Sn oxidation state rises from +2 to +4 (loss of electrons).
• Hg oxidation state falls from +2 to +1 (gain of electrons).
• Cl shows no change, so Sn is oxidized.

Why A is correct:

• Sn²⁺ increases oxidation state from +2 to +4, losing two electrons per the definition of oxidation.

Why the others are wrong:

• B. Cl⁻ maintains -1 oxidation state in all compounds.
• C. Hg²⁺ decreases from +2 to +1 in Hg₂²⁺, indicating reduction.
• D. Hg₂²⁺ forms as the reduced product, not undergoing oxidation.

Final answer: A