To solve this problem, we need to understand the role of cofactors in enzyme activity, specifically in ATP-dependent phosphate transfer reactions.$\newline$Enzymes that utilize ATP for phosphate transfer often require a metal ion as a cofactor.$\newline$The metal ion helps stabilize the negative charges on the phosphate groups of ATP, facilitating the transfer process.$\newline$Among the alkaline earth metals, magnesium (Mg) is the most common cofactor for such reactions.$\newline$Magnesium ions $(Mg^{2+})$ are essential because they:$\newline$• Stabilize the structure of ATP by coordinating with the phosphate groups.$\newline$• Help in the proper orientation of ATP for effective catalysis.$\newline$• Are involved in the activation of the enzyme's active site.$\newline$Analyzing the options:$\newline$• Option 1: Be (Beryllium) is not typically used as a cofactor in biological systems.$\newline$• Option 2: Mg (Magnesium) is the correct cofactor for ATP-utilizing enzymes.$\newline$• Option 3: Ca (Calcium) is involved in other cellular processes but not primarily in ATP phosphate transfer.$\newline$• Option 4: Sr (Strontium) is not commonly used in biological systems as a cofactor.$\newline$Therefore, the correct option is Option 2: Mg.