Answer: Mg2+
Explanation: ATP(Adenosine triphosphate) is unstable in nature and also shows the property of being electronegative. It is seen that [tex]O^-[/tex] is the electronegative element and it is bonded with the Mg2+ being a positive divalent and thus achieve the stability and reducing the electrostatic repulsion.So, there is the combination between the negatively charged phosphate groups with the Mg2+. Therefore option (a) is the correct answer.
