Aquatic plants living in a slightly salty environment such as a lagoon that has a fresh water river that empties in to it live in what type of solution (compared to their cytoplasm) under normal conditions? what happens during an exceptionally rainy year to the solution that those plants live in? what do the plant cells have to do in order to maintain a constant internal environment despite the changing external conditions noted?

The aquatic plants which were living in the slightly saline water the cells, the saline water will actas an hypertonic solution. The water will come out of the cell cytoplasm and cell cytoplasm will get shrink. When such plants with this condition are subjected to fresh water condition, fresh water will act as a hypotonic solution for such plant cells and water will enter inside the plant cell and cytoplasm will get swell.

Exceptionally rainy year will add salts to fresh water again the condition will become hypertonic solution. Water will come out of the cell and cell cytoplasm will get shrink.

This phenomena is known as plasmolysis. Plasmolysis due to changing water and solution conditions may cause injury to the plant cells. Therefore for maintaining internal environment with respect to changing external environment. The mechanism is controlled by turgor pressure which is an hydrostatic pressure that intracellular liquid exerts on the cell membrane.

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link:

c should be the answer