The complex ion that will lose its crystal field stabilization energy upon oxidation of its metal to +3 state is: [Fe(phen)3]2+, [Ni(phen)3]2+, [Co(phen)3]2+, [Zn(phen)3]2+

The correct option is A [Fe(phen)3]2+
By oxidation of Fe2+ into Fe3+, the CFSE value decrease
(2) [Zn(phen)3]2−e−−−−→[Zn(phen)3]3+
Zn2+: 3d10
Zn3+: 3d9
C.F.S.E=0
C.F.S.E=−0.6Δo
By oxidation of Zn2+ into Zn3+, the CFSE value increase
(3) [Ni(phen)3]2+−e−−−−→[Ni(phen)3]3+
Ni2+: 3d8
Ni3+: 3d7
C.F.S.E=−1;.2Δ0
C.F.S.E=−1;.8Δ0
By oxidation of Ni2+ into Ni3+, the CFSE value increase
(4) [Co(phen)3]2+−e−−−−→[Co(phen)3]3+
Co2+: 3d7
Co3+: 3d6
C.F.S.E=−1;.8Δ0
C.F.S.E=−2;.4Δ0
By oxidation of Co2+ into Co3+, the CFSE value increase.