Final answer:
The force that supports plant cell structure against gravity is called turgor pressure, which results from the osmotic influx of water into cells making them rigid. This pressure is vital for plant rigidity and to prevent wilting.
Step-by-step explanation:
The force that pushes plant cell membranes tightly against the cell wall and helps support softer tissues against the force of gravity is known as turgor pressure. The correct answer to the question is option b) Turgor. Turgor pressure in plants arises when water moves into a plant cell in a hypotonic environment, causing an osmotic pressure that raises the cell's internal pressure until no more water can enter. This creates stiffness in the cell walls, supporting the plant structure.
Osmotic pressure causes water to flow into the plant cells, making them turgid, which is crucial for maintaining plant rigidity and preventing wilting. In contrast, when plants are not watered sufficiently, they experience plasmolysis, where the cell membrane detaches from the cell wall due to loss of water.