b
\(\mathrm{H}_{2} \mathrm{O}\) is a weak field ligand, hence

\(\Delta_{o}<\) pairing energy.

\(\mathrm{CFSE}=(-0.4 x+0.6 y) \Delta_{o}\)

where, \(x\) and \(y\) are no. of electrons occupying \(t_{2 g}\) and es orbitals respectively.

For \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) complex ion.

\(\mathrm{Fe}^{3+}\left(3 d^{5}\right) =t_{2 \mathrm{g}}^{3} e_{\mathrm{g}}^{2}=-0.4 \times 3+0.6 \times 2\)

\(=0.0 \text { or } 0\; \mathrm{Dq}\)