Ce⁴⁺ < Pb⁴⁺ < Bi³⁺ < Co³⁺
Co³⁺ < Pb⁴⁺ < Ce⁴⁺ < Bi³⁺
Co³⁺ < Ce⁴⁺ < Bi³⁺ < Pb⁴⁺
Bi³⁺ < Ce⁴⁺ < Pb⁴⁺ < Co³⁺
The correct option is D Bi³⁺ < Ce⁴⁺ < Pb⁴⁺ < Co³⁺
As the reduction potential (E⁰red) increases, oxidizing power also increases. So, the correct order is: Bi³⁺ < Ce⁴⁺ < Pb⁴⁺ < Co³⁺.