d
\(Cl ^{-}\)is a weak ligand while \(CN ^{-}\)is a strong ligand.

In \(\left[ MnCl _4\right]^2: d^5\) configuration and \(5\) unpaired electrons

In \(\left[ CoCl _4\right]^2: d^7\) configuration and \(3\) unpaired electrons

In \(\left[ Fe ( CN )_6\right]^4: d ^6\) configuration, low spin complex and o unpaired electrons

More the number of unpaired electrons, more the value of magnetic moment.

Hence, the order is \(\left[ MnCl _4\right]^{2-} > \left[ CoCl _4\right]^{2-} > \left[ Fe ( CN )_6\right]^{4-}\)