Concept:Chemical properties depend on the number of valence electrons.
Explanation:Alkaline earth metals belong to Group 2 of the periodic table.
Each of them has two valence electrons in its outermost shell.
Elements having the same number of valence electrons show similar chemical behaviour.
As a result, they form ions with the same charge and undergo similar reactions.
Being in the same period is not the reason, because period indicates the number of electron shells, not the valence electrons.
Colourless salts and high reactivity are consequences of their electronic structure, not the cause of their similarity.
Hence, the similarity arises from the same number of valence electrons.
Answer:C. They have the same number of valence electrons