The pair in which both species have same magnetic moment (spin only value) is

Step-by-step solution

Spin only magnetic moment, _s= n(n+2) where n= number of unpaired electrons. Number of unpaired electrons in Cr ²⁺ ([ Ar ] 3 d^4 ) is 4 , in Co ²⁺ ([ Ar ] 3 d^7 ) is 3 , in Fe ²⁺ ([ Ar ] 3 d^6 ) is 4 , in Mn ²⁺ ([ Ar ] 3 d^5 ) is 5 . As the number of unpaired electrons in Cr ²⁺ and Fe ²⁺ are same, hence [ Cr ( H ₂ O )₆ ]²⁺ and [ Fe ( H ₂ O )₆ ]²⁺ will have same magnetic moment.