The electronic configuration of cobalt ($Co$,atomic number $27$) is:

• A
  $1s^2, 2s^2 2p^6, 3s^2 3p^6 3d^1, 4s^2$
• B
  $1s^2, 2s^2 2p^6, 3s^2 3p^6 3d^7, 4s^2$
• C
  $1s^2, 2s^2 2p^6, 3s^2 3p^6 3d^3, 4s^2$
• D
  $1s^2, 2s^2 2p^6, 3s^2 3p^6 3d^5, 4s^2$