The correct answer is D. Osmotic problems.
Freshwater animals have evolved to live in an environment with a low salt concentration. Their cells have a high concentration of salts, and they need to take in water from their environment to maintain this balance. If freshwater animals were to live in seawater, the water would flow out of their cells, causing them to dehydrate and die.
Seawater animals, on the other hand, have evolved to live in an environment with a high salt concentration. Their cells have a low concentration of salts, and they need to get rid of excess water from their environment to maintain this balance. If seawater animals were to live in freshwater, the water would flow into their cells, causing them to swell and die.
Osmotic problems are the main reason why freshwater animals cannot live for long in seawater and vice versa.
Option A is incorrect because the levels of nitrogen are not significantly different in freshwater and seawater.
Option B is incorrect because the levels of thermal tolerance are not significantly different in freshwater and seawater.
Option C is incorrect because the variations in light intensity are not significantly different in freshwater and seawater.
Option E is incorrect because the spectral quality of solar radiation is not significantly different in freshwater and seawater.