Introduction:
The square planar geometry is a type of molecular geometry that describes the arrangement of atoms in a molecule. In this geometry, the central atom is surrounded by four ligands that are arranged in a square plane around it. The square planar geometry is commonly observed in transition metal complexes and some organic molecules.

Explanation:
The correct answer to the question is option 'A', which states that the square planar geometry is based on the Z-exclusion principle. Let's understand why this is the correct answer.

Z-exclusion principle:
The Z-exclusion principle, also known as the 18-electron rule, is a concept in chemistry that describes the stability of transition metal complexes based on the number of valence electrons present. According to this principle, stable transition metal complexes tend to have 18 valence electrons surrounding the central metal atom.

Electronic configuration:
In the square planar geometry, the central atom is typically a transition metal, which has d orbitals available for bonding. The electronic configuration of transition metals allows them to form coordination complexes by donating and accepting electron pairs.

Ligands in square planar geometry:
In a square planar geometry, the central atom is surrounded by four ligands arranged in a square plane. These ligands can be either negatively charged (anions) or neutral molecules. The ligands form coordinate covalent bonds with the central atom by donating a pair of electrons.

Electronegativity of ligands:
The Z-exclusion principle is based on the fact that the ligands in a transition metal complex are negatively charged or neutral. This is because ligands with positive charges would be highly electronegative and would not form stable bonds with the metal atom. Hence, the Z-exclusion principle ensures that the ligands are electroneutral or negatively charged to maintain stability.

Conclusion:
In conclusion, the square planar geometry is based on the Z-exclusion principle, which describes the stability of transition metal complexes based on the number of valence electrons present. This principle ensures that the ligands surrounding the central metal atom in a square planar geometry are electroneutral or negatively charged, maintaining stability in the complex.