The electronic configuration of calcium ion $(Ca^{2+})$ is

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