Four successive members of the first row transition elements are listed below with atomic numbers. Which one of them is expected to have the highest E°M3+/M2+ value?

SRP value normally increases from left to right in the period of d-block elements. Some SRP value are exceptionally higher due to the stability of product ion. The standard reduction potential (SRP) values are related to the ease of reduction of a metal ion. A higher SRP value indicates that the M3+ ion is more readily reduced to M2+. In the case of the first-row transition elements, the SRP values generally increase across the period, but there are exceptions due to the relative stability of the different oxidation states. For example, Mn3+ has a relatively high SRP because Mn2+ is more stable than Mn3+. Similarly, the SRP value for Co3+/Co2+ is higher than Fe3+/Fe2+ or Cr3+/Cr2+ because Co2+ is more stable than Co3+. Therefore, the element with the highest E°M3+/M2+ value among the given options is expected to be Co(Z=27) due to the exceptional stability of Co2+ compared to Co3+.