Square Planar Complex:
In a square planar complex, the central metal ion is surrounded by four ligands in a square plane. The coordination number of the metal ion is 4.

To form a square planar complex, a minimum of 4 monodentate ligands is required. Each ligand contributes one electron pair to form a coordinate bond with the metal ion.

The ligands bind to the metal ion through a coordinate bond, where the ligand donates a pair of electrons to the metal ion. Monodentate ligands have only one donor atom, and they can form one coordinate bond with the metal ion. Therefore, each ligand can contribute one electron pair to the complex.

Tetrahedral Complex:
In a tetrahedral complex, the central metal ion is surrounded by four ligands in a tetrahedral arrangement. The coordination number of the metal ion is 4.

To form a tetrahedral complex, a minimum of 4 monodentate ligands is required. Each ligand contributes one electron pair to form a coordinate bond with the metal ion.

Similar to square planar complexes, each monodentate ligand can form one coordinate bond with the metal ion in a tetrahedral complex. Therefore, a minimum of 4 monodentate ligands is required to form a tetrahedral complex.

Conclusion:
In both square planar and tetrahedral complexes, a minimum of 4 monodentate ligands is required to satisfy the coordination number of the metal ion. Each monodentate ligand contributes one electron pair to form a coordinate bond with the metal ion. Therefore, the correct answer is option C) 4.