How is the $t_{2g}^4 e_g^0$ configuration possible for a $d^4$ ion during crystal field splitting in an octahedral complex?

$A$ $[M(H_2O)_6]^{2+}$ complex typically absorbs at around $600 \ nm$. It is allowed to react with ammonia to form a new complex $[M(NH_3)_6]^{2+}$ that should have absorption at ....... $nm$.

The electron distribution in $d^{n}$ coordination complexes depends on the magnitude of crystal field splitting,$(\Delta_{0})$ and pairing energy $(P).$ The condition which favours the formation of high spin complexes is

Identify the correct increasing order of field strength of ligands from the following.

The complex with the highest magnitude of crystal field splitting energy $\left(\Delta_0\right)$ is

What is the spectrochemical series? Explain the difference between a weak field ligand and a strong field ligand.