What is the reason of paramagnetism in transition metals ?

Explanation

The electronic configuration of Zn (Z = 30) is `Zn = [Ar]3d^(10) 4s^(2)`
The electronic configuration of `Zn^(2+) ` is `Zn^(2+) = [Ar] 3d^(10)`
Since, the 3d- orbital in `Zn^(2+)` are completely filled (no unpaired electrons),
`:. Zn^(2+)` is diamagnetic in nature.