Answer: Option (b) is the correct answer.
Step-by-step explanation:
Elements which belong to the same group tend to show same chemical properties because elements present in the same group have same number of valence electrons.
An alkali metal will show an oxidation state of +1. This is because alkali metals contain only one valence electron and to attain stability they lose one electron.
So, the option A is an alkali metal; B forms a 1- ion, is incorrect.
When magnesium loses its 2 valence electrons then it acquires +2 charge and oxygen atom on gaining these 2 electrons will acquire a -2 charge resulting in the formation of MgO.
Therefore, the option A has two valence electrons; B forms a 2- ion, is correct.
Noble gases have already completely filled orbital so, they do not lose or gain electrons easily.Hence, the option A is a noble gas; B has seven valence electrons, is incorrect.
Also, the option A forms a -3 ion; B has five valence electrons, is incorrect.
Thus, we can conclude that the pair of elements in which A has two valence electrons; B forms a 2- ion, could possibly be found in the same group on the periodic table.