A coordination number of four can lead to either tetrahedral or square planar geometry, and the distinction is reflected in the hybridisation of the metal. Square planar complexes use dsp^2 hybridisation, which combines one (n-1)d orbital, one ns orbital and two np orbitals to give four hybrid orbitals directed to the corners of a square. This arrangement, typical of d^8 ions like Ni(II), Pd(II) and Pt(II) with strong-field ligands, forces electron pairing and commonly produces diamagnetic complexes. The sp^3 option gives tetrahedral geometry and usually leaves unpaired electrons, so it does not match a diamagnetic square planar shape. sp^3d corresponds to a five-coordinate trigonal bipyramidal arrangement, not four. d^2sp^3 is octahedral and six-coordinate. The link between dsp^2 hybridisation and square planar geometry is exactly as presented in NCERT valence bond discussions. Plausibility check: four square-directed hybrids with all electrons paired neatly explain both the geometry and the observed diamagnetism, so the answer is consistent.