I believe it is Magnesium. This is because a substance will need more energy if it is harder to split up or in other words has less spacing and higher attraction. With its little # of E levels, the electrons in the outer valence E level has a higher attraction to the protons in the nucleus than the Iron and Lead. Then since Mg has paired electrons in its S energy level while Aluminum only lone electron, since paired electrons make a substance more stable, it’ll need more energy than Aluminum to split them apart.