Whats the electronic configuration of the copper ion Cu2+

Does anyone know for sure, a few answers came up some being:

[Ar]3d74s2

[Ar]4s0 3d9

[Ar] 3d10 4s2 4p1

Any know for sure which one is correct and why?

These maybe wrong above, not positive.

Definitely [Ar]3d9. The 4s electrons are always lost first when a transition metal ion (of Period 4) is formed.

[Ar] 4s2 3d9