The ground state electronic configuration of Cu is

The correct answer is: [Ar] 4s¹ 3d¹⁰

 

The atomic number of copper = 29
[Ar] = 1$s^2$ 2$s^2$ 2$p^6$ 3$s^2$ e$p^2$
Hence, $C{u}_{29}$ = [Ar]3$d^{10}$ 4$s^1$
The ground state electronic configuration of an element's atom always corresponds to the lowest overall electronic energy state. In the case of Cr and Cu, when the energies of the two subshells (4s and 3d) differ somewhat, an electron changes from a lower energy (4s) to a higher energy (3d), given that such a shift results in all orbitals of the higher energy subshell being either totally filled or half filled.