Which one of the following is the activator for enzyme hexokinase...

Hexokinase is the first enzyme in the glycolysis pathway, which catalyzes the phosphorylation of glucose to glucose-6-phosphate using ATP.

The true substrate for hexokinase is not free ATP, but the $\text{Mg}^{++}$-ATP complex. The $\text{Mg}^{++}$ ion shields the negative charges of the phosphate groups in ATP, making the terminal phosphorus atom an easier target for nucleophilic attack by the hydroxyl group of glucose.

Therefore, $\text{Mg}^{++}$ acts as an essential cofactor and activator for the enzyme hexokinase.

Answer: $\text{Mg}^{++}$