Which of the following groups have one valence electron and are very reactive? column 1 alkali metals column 2 alkaline earth metals column 17 halogens column 18 noble gases

Column 1 (IA) in the periodic table which is named as alkali metals contains elements which have only one valence electron (nS¹) in the last orbital that can be easy to lose it during the chemical reaction and they are very reactive.  

The alkali metals have a higher atomic radius than alkaline earth metals. Even more alkali metals have to lose only one electron, while alkaline earth metals will find it relatively harder to lose the second electron. Hence alkali metals are more reactive than alkaline earth metals. The alkali earth metals have two valence electrons.  

The noble gases are chemically inert, due that their orbitals are fully occupied by electrons.

The halogens are reactive but they have seven valence electrons not one as the question ask.

it means there are 3 atoms of oxygen