What is the ground state electronic configuration of a ${ }_{27}\mathrm{Co}$ atom in the gas phase?

Ground state electronic configuration of a ${ }_{27}\mathrm{Co}$ atom in the gas phase is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{7}4s^2$.

The number of electrons in each sublevel and energy level of the ground state atom is displayed in the electron configuration of an atom.

Hence, option A is correct.