Plants and animals that live in estuaries face a dilemma in maintaining water balance inside their cells. During high tide they are in salt water, while at low tide they are in fresh water. At high tide a plant or animal cell will be in .

The correct answer is "a hypertonic solution".

Explanation:

Seawater is the most clear example of a hypertonic solution in nature. The concentration of ions in seawater are far more superior than the concentration of ions inside a plant or an animal cell, since seawater have an osmolarity of about 1000 mOsm/l. Therefore, at high tide a plant or animal cell will be in a hypertonic solution, and the cells must have adaptions to avoid cell shrinking and dead.