Answer:
Hypertonic.
Explanation:
When salt water invades fresh water the plants are killed because the salt water solution is hypertonic to the plants, drawing water from their cells. Cells are surrounded by solutions. The solution inside the cell is surrounded by a permeable membrane. This membrane divides the solution inside and the solutions outside the cell. A hypertonic solution has more solutes than those of the cell so the osmotic pressure will draw water from the cell and the cells of the plant will shrink and die.
