1. Identify the metal ion configuration

For $\mathrm{Co^{3+}}$:

So all statements must be checked for an octahedral or tetrahedral $d^6$ system.

2. Check statement (a)

Octahedral Co(III) complexes with strong field ligands have very high magnetic moments.

For octahedral $d^6$ with strong field ligands, electrons pair up in $t_{2g}$:

This is a low-spin configuration with no unpaired electrons. Hence magnetic moment is very low (actually diamagnetic, $\mu \approx 0$ BM), not very high.

So, (a) is incorrect.

3. Check statement (b)

When $\Delta_o < P$, the d-electron configuration of Co(III) in an octahedral complex is $t_{eg}^4 e_g^2$.

There is a typographical issue in the statement: in octahedral splitting, the lower set is $t_{2g}$, not $t_{eg}$. Interpreting it as the intended configuration:

If $\Delta_o < P$, the complex is high spin. For octahedral $d^6$ high-spin configuration:

That is correct for $d^6$.

So (b) is correct (clearly intended as $t_{2g}^4 e_g^2$).

4. Check statement (c)

Wavelength of light absorbed by $[Co(en)_3]^{3+}$ is lower than that of $[CoF_6]^{3-}$.

Use spectrochemical series:

Thus,

Energy absorbed is proportional to $\Delta_o$:

Larger $\Delta_o$ means larger energy, hence smaller wavelength. Therefore,

So (c) is correct.

5. Check statement (d)

If the $\Delta_o$ for an octahedral complex of Co(III) is $18000\,\text{cm}^{-1}$, the $\Delta_t$ for its tetrahedral complex with the same ligand is $16000\,\text{cm}^{-1}$.

For the same metal ion and same ligand:

So,

Not $16000\,\text{cm}^{-1}$.

So (d) is incorrect.

6. Conclusion

Incorrect statements are:

Therefore, the correct option is D.