Answer:
Magnesium is the correct answer
Step-by-step explanation:
It is magnesium because as we move down the periodic table group, atomic sizes increase which is as a result of lesser forces of attraction between nucleus and valence of an atom which makes the atom to be prone to leave as electrons so easily, elements become more reactive.
But as we move along the period , atomic sizes decrease because there is a more force of attraction between nucleus of an atom and it valence electrons making it less reactive just as the case in magnesium. Other elements are more reactive than it across the group.