The correct answer is Hypotonic.
When a plant cell is placed in a hypotonic solution, it becomes firmer and more rigid. A hypotonic solution is the one, which contains less concentration of solute and more concentration of solvent ( water) outside the cell as compared to inside.
When a plant cell is placed in this solution, water moves from the solution to inside of the cell. This is because through osmosis, water always move from its high concentration to low concentration. Due to this, turgor pressure will develop in the plant cell and it will become firmer and more rigid.