Which one of the following is diamagnetic?

A magnetic field repels diamagnetic materials because it induces an opposing magnetic field in them when it is applied, which produces a repelling force.

If all the electrons are paired or if none of the species have unpaired electrons, then we can determine that element as a diamagnetic material. Since, out of all the elements, only Sc³⁺ has zero unpaired electrons, its diamagnetic. 

Co²⁺= [Ar]3d⁷  =3 unpaired electrons

Cu²⁺= [Ar]3d⁹=1 unpaired electron

Mn²⁺=[Ar]3d⁵=5 unpaired electron

Sc³⁺=[Ar]3d⁰=0 unpaired electron

Therefore, in the given elements Sc³⁺ is diamagnetic. 

Hence , the correct option (D).