Answer:
Barium (Ba)
Explanation:
Barium is a group 2 elements.
Valence shell electronic configuration =
Its valence shell has only two electrons and by loosing these electrons, it attains inert gas configuration. therefore, it looses electron easily.
It is highly reactive and because of its high reactive nature, never present in nature in free state.
In general, reactivity increases down the group in the periodic table.
This is because down the group atomic size increases and electrons becomes far from the nucleus, so outer electrons are not tightly held by the nucleus. Therefore, valence shell electrons loose easily which in turn increases reactivity.
So, Ba is more reactive among all the given elements.