The pair which both the species have the same magnetic moment (spin only) is :

Options

A. a.`[Cr(H_(2)O)_(6)]^(2+),[CoCl_(4)]^(2-)`
B. b.`[Cr(H_(2)O)]^(2+),[Fe(H_(2)O)_(6)]^(2+)`
C. c.`[Mn(H_(2)O)]^(2+),[Cr(H_(2)O)_(6)]^(2+)`
D. d.`[CoCl_(4)]^(2-),[Fe(H_(2)O)_(6)]^(2+)`

Answer

B

Explanation

Same magnetic moment =Same number of unpaired electrons
`mu=sqrt(n(n+2))`
where `n=` number of unpaired electrons
`Co^(2+) = 3d^(7), 3` unpaired electrons
`Cr^(2+) = 3d^(4), 4` unpaired electrons,
`Mn^(2+) = 3d^(5),5` unpaired electrons
`Fe^(2+) =3d^(6),4` unpaired electrons.