Answer:
Co⁰ - 3e⁻ → Co³⁺ (cobalt loses electrons so it is oxidized)
Step-by-step explanation:
When a substance is oxidized it means that it loses electrons.
When a substance is reduced it means that it gains electrons.
Ni²⁺ + 2e⁻ → Ni⁰ (nickel gains electrons so it is reduced)
Co⁰ - 3e⁻ → Co³⁺ (cobalt loses electrons so it is oxidized)