Magnetic moment (µ) is calculated as follows: µ=√n(n+2).BM where, n= number of unpaired electrons.
(d) CO2+=6+8−2=12 electrons. Thus, has 4 electrons in 2p-subshell with 2-unpaired electron. ∴µ=√2(2+2)=√8 Hence, Mn2+ has highest magnetic moment and option (b) is the correct answer.