Final answer:
Sodium fluoride is the additive used to prevent glycolysis by acting as an inhibitor of glycolytic enzymes, preserving blood glucose levels in samples.
Step-by-step explanation:
The additive that provides a physical barrier to prevent glycolysis is sodium fluoride. When collecting blood samples for glucose measurement, it is crucial to inhibit glycolysis so that the glucose levels in the blood don't continue to decrease after the sample has been taken. This is due to the fact that glycolysis can continue in blood cells, leading to an underestimation of the patient's true blood glucose level. Sodium fluoride acts as an inhibitor of the enzymes that catalyze the glycolysis pathway, thus preserving the glucose concentration. Therefore, the correct answer is (d) Sodium fluoride.