Which type of solution would most likely cause a plant cell placed in it to become firmer and more rigid?

Contrast with other solutions: In a hypertonic solution, water moves out of the cell, causing it to shrink and become flaccid. In an isotonic solution, there is no net movement of water, so the cell remains unchanged in firmness.

Relate to plant cell structure: Plant cells have a rigid cell wall that prevents them from bursting when water enters. The influx of water in a hypotonic solution fills the central vacuole, pushing the plasma membrane against the cell wall and increasing rigidity.