The pair in which both the species have same magnetic moment (spin only) is.A     [𝐶𝑜𝐶𝑙4]2−,  [𝐹𝑒(𝐻2𝑂)6]2−B     [𝑀𝑛(𝐻2𝑂)6]2+ , [𝐶𝑟(𝐻2𝑂)6]2+C     [𝐶𝑟(𝐻2𝑂)6]2+, [𝐹𝑒(𝐻2𝑂)6]2+D

The magnetic moment of a complex is determined by the number of unpaired electrons in the d-orbitals of the central metal atom. The formula to calculate the magnetic moment (spin only) is √n(n+2) where n is the number of unpaired electrons.

Let's calculate the magnetic moment for each species:

A. [CoCl4]2− and [Fe(H2O)6]2− Co in [CoCl4]2− is in +2 oxidation state (d7 configuration) and has 3 unpaired electrons. Fe in [Fe(H2O)6]2− is also in +2 oxidation state (d6 configuration) but has 4 unpaired electrons. So, they have different magnetic moments.

B. [Mn(H2O)6]2+ and [Cr(H2O)6]2+ Mn in [Mn(H2O)6]2+ is in +2 oxidation state (d5 configuration) and has 5 unpaired electrons. Cr in [Cr(H2O)6]2+ is in +2 oxidation state (d4 configuration) and has 4 unpaired electrons. So, they have different magnetic moments.

C. [Cr(H2O)6]2+ and [Fe(H2O)6]2+ Cr in [Cr(H2O)6]2+ is in +2 oxidation state (d4 configuration) and has 4 unpaired electrons. Fe in [Fe(H2O)6]2+ is in +2 oxidation state (d6 configuration) and also has 4 unpaired electrons. So, they have the same magnetic moments.

Therefore, the pair in which both the species have the same magnetic moment (spin only) is C. [Cr(H2O)6]2+ and [Fe(H2O)6]2+.