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

In 2+ 2 6 [Cr(H O) ] , the chromium metal is present in the oxidation state of +2 and there are 4 unpaired electrons. Similarly, in 2+ 2 6 [Fe(H O) ] the iron metal is present in the oxidation state of +2 having 4 unpaired electrons. Hence, both the complex will have the same magnetic moment. Hence, the correct answer is option (a)