The solution most likely to cause a plant placed in it to become firmer and more rigid is a hypotonic solution.
Explanation
- A hypotonic solution has a lower concentration of solutes outside the plant cells compared to inside. This causes water to move into the cells by osmosis.
- As water enters, plant cells become turgid (swollen), which makes the plant firmer and more rigid due to increased turgor pressure in the cell walls.
- This turgidity is crucial for maintaining the structural integrity and rigidity in plants.
This contrasts with hypertonic solutions, which cause cells to lose water and shrink, leading to wilting rather than firmness. Isotonic solutions result in no net water movement, so the firmness would not change significantly.