Electronic configuration of 3d series

Which of the following is general is general electron configuration of 4d series?

Electronic configuration of M^(3+) is [Ar]3d^(10)4s^(2) , it belongs to

The electronic configuration of copper is [Ar] 4s^(1) 3d^(10) and not [Ar] 4s^(2) 3d^(9) . Justify.

The electronic configuration of valence shell of Cu is 3d^(10)4s^(2) . How is this configuration explained ?