Model A represents an atom that is more reactive than the others represented.
Valence electrons actually determine the reactivity of elements. They also determine the properties of elements.
Elements with one valence electron are highly reactive because they need low energy to remove them. They can either gain more electrons to become stable or they share/give out their electrons.
Therefore, Model A is the correct answer because it has one valence electron and its valence electron is farther from the nucleus thereby this makes it more reactive.
