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

Hypotonic solution

Explanation:

A hypotonic solution is one with lower solute concentration compared to the cell sap of a cell suspended in it. Such solution has more water potential compared to the cell in it. Hence, water molecules will move from the solution into the cell, making the cell to be turgid.

If a plant is placed in a hypotonic solution, water will enter into the cells of the plant and makes the plant to be firmer and more rigid due to turgidity.