b
If spin only magnetic moment of the complex is \(4.90 BM ,\) it means number of unpaired electrons should be \(4.\)

\((A)\) In octahedral complex: \(\left[ M \left( H _{2} O \right)_{6}\right]^{2+}\)

\(C.F.S.E.\) \(=\left(-0.4 \Delta_{0}\right) \times 4+\left(+0.6 \Delta_{0}\right) \times 2+0 \times P\)

\((B)\) In tetrahedral complex: \(\left[ M \left( H _{2} O \right)_{4}\right]^{2+}\)

\(C.F.S.E.\) \(=\left(-0.6 \Delta_{t}\right) \times 3+\left(+0.4 \Delta_{t}\right) \times 3+0 \times P\)

\(=-0.6 \Delta_{ t }\)