In the lumen of the proximal tubule, Na concentration is the Na concentration inside the cells of the tubule wall.

The correct answer is - higher than

Explanation:

In the lumen of the proximal tubule of the kidney, most of the solutes are absorbed in the form of sodium. There is a gradient of sodium ions which allows the absorption of the solutes from the lumen into the cell.

It is due to the solute or  Na concentration is higher in the lumen than Na concentration inside the cells of the tubule wall. So movement of the solutes is due to the concentration gradient.