The electronic configuration of $\ce{Fe}$ ($Z = 26$) is:

• a$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^6\,4s^2$
• b$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^2\,3d^8$
• c$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^8$
• d$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^2\,4p^6$