Q.Which one of the following is diamagnetic
a. Cr2⊕
b. Fe3⊕
c. Cu2⊕
d. Sc3⊕

Every unpaired electron in an atom or ion contributes a small magnetic moment. For first-row transition-metal ions, where orbital angular momentum is largely quenched by the surrounding environment, the magnetic moment is well described by the spin-only formula, mu = sqrt(n(n+2)) Bohr Magnetons (BM), where n is the number of unpaired electrons -- so measuring a species' magnetic moment lets its unpaired-electron count (and often its oxidation state) be worked out. A substance with every electron paired shows no such moment and is diamagnetic (repelled by an applied magnetic field, e.g. Sc3+ or Zn2+, both d0/d10 with zero unpaired electrons); a substance with one or more unpaired electrons is paramagnetic (attracted into an applied field, e.g. Cu2+, 3d9, one unpaired electron). Iron, cobalt and nickel go further and are ferromagnetic: their unpaired electrons all align in the same direction under an external field, hugely enhancing magnetic susceptibility, and the metal can retain a permanent magnetic moment even after the field is removed. In practice, measured magnetic moments are usually taken in solution or the solid state (where the metal is typically hydrated/ligand-bound), so small differences from the simple spin-only calculated value are commonly seen.