The pair having the same magnetic moment is [At. No.: Cr = 24, Mn = 25, Fe = 26, Co = 27]

Step-by-step solution

In option A: C o C l 4 2 - , C o 2 + 3 d 7 with W.F.L., and [ F e ( H 2 O ) 6 ] 2 + , F e 2 + ( 3 d 6 ) with W.F.L., In option B: [ C r ( H 2 O ) 6 ] 2 + , C r 2 + ( 3 d 4 ) with W.F.L., and [ C o C l 4 ] 2 - , C o 2 + ( 3 d 7 ) with W.F.L., In option C: [ C r ( H 2 O ) 6 ] 2 + , C r 2 + ( 3 d 4 ) with W.F.L., and [ F e ( H 2 O ) 6 ] 2 + , F e 2 + ( 3 d 6 ) with W.F.L., Here both complexes have same unpaired electrons i.e. = 4 In option D: [ M n ( H 2 O ) 6 ] 2 + , M n 2 + ( 3 d 5 ) with W.F.L., and [ C r ( H 2 O )