Vanadium oxidation state unchanged at +5

Steps:

• In NH4VO3, NH4^+ has +1 charge, O3 contributes -6, so V = +5 to balance.
• In VO2^+, two O contribute -4, ion charge +1, so V = +5.
• Subtract initial from final oxidation state: +5 - +5 = 0.
• Chloride ions likely acidify without reducing V.

Why B is correct:

• Per oxidation state rules, V is +5 in both species (NH4^+ +1, 3O^{-2} = -6 for NH4VO3; 2O^{-2} = -4, +1 charge for VO2^+).

Why the others are wrong:

• A: No reduction to +4 (VO^{2+}); solution is V(V) with Cl^- present.
• C: No oxidation; state fixed at +5.
• D: No two-electron loss; unchanged.

Final answer: B