Answer:
(D) 4s
Explanation:
The electronic configuration of a Fe atom is:
1s² 2s² 2p⁶ 3s² 3p6 3d⁶ 4s².
The subshell located farthest from the nucleus is 4s, that's why it experiences the lowest effective nuclear charge.
This can be seen when looking at the electronic configurations of Fe⁺² and Fe⁺³:
Fe⁺² ⇒ 1s² 2s² 2p⁶ 3s² 3p6 3d⁶.
Fe⁺³ ⇒ 1s² 2s² 2p⁶ 3s² 3p6 3d⁵.
