Final answer:
Placing onion cells in a 10% saline solution creates a hypertonic environment, which causes the cells to lose water and shrink due to plasmolysis.
Step-by-step explanation:
When onion cells, which contain 95% water, are placed in a 10% saline solution, the environment outside the cell has a higher concentration of solute (salt) compared to the inside of the cell. This environment is known as a hypertonic solution. In a hypertonic solution, water moves out of the cells to balance the concentration of solutes inside and outside. This leads to a decrease in the cell's volume as the cell loses water, a process called plasmolysis where the plasma membrane pulls away from the cell wall. Therefore, you would expect the onion cell to shrink as the correct answer is indeed (a) The cell to shrink.