Answer : Option A) Sodium
Explanation : Sodium is the element which is most reactive tin the given options.
Sodium has the atomic number as 11, which is distributed in the atomic shells as 2-8-1.
So, it is clear that the outermost valence shell contains only one electron in sodium. To give away one electron and attain the octet state is easy for sodium. If it donates the electron it becomes a cation which is very reactive, as it gets a positive charge on itself.
Compared to the valency of the other atoms given here, Nickel has 2 valence electrons so it will be less reactive than Sodium.
Carbon has 4 valence electrons which is less than sodium and oxygen needs 2 electrons to be an octet which is also less.
