The correct option is D Co2+
Cr3+ has 3 unpaired electrons,
Magnetic moment = 3.87 BM.
Mn2+ has 5 unpaired electrons,
Magnetic moment = 5.91 BM.
Cr2+ has 4 unpaired electrons,
Magnetic moment = 4.89 BM.
Co2+ has 3 unpaired electrons,
Magnetic moment = 3.87 BM.