Correct Answer - D
`._26Fe=[Ar]3d^6, 4s^2 rArr ` No. of d electron =6
`Fe^(2+)=[Ar]3d^6 , 4s^0 rArr` No. of d electron =6
`._10Ne=1s^2 2s^2 2p^6 rArr ` No. of p electron =6
`._12Mg=1s^2 2s^2 2p^6 3s^2 rArr` No. of s electron = 6
`Cl^(-)=1s^2 2s^2 2p^6 3s^2 3p^6 rArr` No. of p electron =12
Hence the ans. is `Cl^-`