Hypotonic Solution:
When a cell is placed in a hypotonic solution, it means that the concentration of solutes outside the cell is lower than the concentration inside the cell. In this situation, water molecules move into the cell in an attempt to equalize the concentration on both sides of the cell membrane. This influx of water causes the cell to swell up.

Hypertonic Solution:
In contrast, when a cell is placed in a hypertonic solution, it means that the concentration of solutes outside the cell is higher than the concentration inside the cell. In this case, water molecules move out of the cell to dilute the higher solute concentration outside. As a result, the cell loses water and shrinks.

Isotonic Solution:
An isotonic solution refers to a solution with the same concentration of solutes as that of the cell. In this scenario, there is no net movement of water across the cell membrane, and the cell maintains its normal shape and size.

Explanation:
When a cell is placed in a hypotonic solution, the concentration of solutes outside the cell is lower than that inside the cell. Due to the process of osmosis, water molecules move from an area of higher concentration (inside the cell) to an area of lower concentration (outside the cell). This movement of water into the cell causes it to swell up or expand.

The reason behind the cell swelling in a hypotonic solution is that the higher solute concentration inside the cell creates an osmotic pressure that draws water into the cell. As more and more water enters the cell, its volume increases, leading to swelling.

On the other hand, when a cell is placed in a hypertonic solution, the concentration of solutes outside the cell is higher than inside the cell. In this case, water molecules move out of the cell to dilute the higher solute concentration outside. This loss of water from the cell causes it to shrink or shrink.

In an isotonic solution, the concentration of solutes is equal both inside and outside the cell. As a result, there is no net movement of water across the cell membrane, and the cell maintains its normal shape and size.

Therefore, the correct answer is option 'A' - a cell swells up when kept in a hypotonic solution.

A cell swells up when kept in a hypotonic solution due to the process of endosmosis. Endosmosis is the movement of water into the cell and such a plant cell is referred to as turgid.