Reduction is gain of electrons or decrease in oxidation state

Steps:

• Assign oxidation states: Ti in TiO₂ is +4, O is -2; Cl in Cl₂ is 0.
• In products: Ti in TiCl₄ is +4, Cl is -1, O in O₂ is 0.
• Chlorine changes from 0 to -1, a decrease indicating reduction.
• Oxygen changes from -2 to 0, an increase indicating oxidation; Ti unchanged.

Why B is correct:

• Chlorine gains electrons as its oxidation state decreases from 0 in Cl₂ to -1 in TiCl₄, per the definition of reduction.

Why the others are wrong:

• A. Carbon does not appear in the reaction.
• C. Oxygen is oxidized, with oxidation state increasing from -2 to 0.
• D. Titanium's oxidation state remains +4 in both reactant and product.

Final answer: B