Concept:Osmosis is the movement of water molecules across a semi-permeable membrane from a region of higher water potential to a region of lower water potential.
Explanation:In root hair cells, water enters through the cell membrane by osmosis.
Cell sap, carbon dioxide, and oxygen do not move mainly by osmosis.
Cell sap is the solution inside the vacuole, not a substance entering the cell.
Carbon dioxide and oxygen diffuse through the membrane along their concentration gradients.
Therefore, water is the substance that passes through the root cell membrane by osmosis.
Answer:D. Water