The correct answer is: sodium gradient
It is very important for glucose to be transported into proximal tubule cells (reabsorbed) because that is the way to prevent it from disappearing from the body through the urine.
Glucose is co-transported with sodium ions into the proximal tubule via the SGLT2 cotransporter. This cotransporter is an example of secondary transport because it uses the energy from downhill sodium ion gradient created by the ATPase pump to transport glucose.