Which of the following configurations is correct for iron $(Fe)$?

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