Which of the following equations shows the greatest change in oxidation state for the oxidation of the reactant?

(a) Cu(s) → Cu²⁺(aq) + 2e⁻
(b) Cu²⁺(aq) + 2e⁻ → Cu(s)
(c) Cu²⁺(aq) → Cu(s) + 2e⁻
(d) Cu⁺(aq) → Cu²⁺(aq) + e⁻

Cu has oxidation state of 0, while Cu²⁺ has oxidation state of +2.