Fresh water animals cannot live for long in sea water and vice versa as they face:

Osmotic problems

The correct answer is Option (4) - Osmotic problems

Osmosis is the movement of water across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration. Freshwater animals are adapted to living in environments where the surrounding water has a much lower concentration of solutes (such as salts) than their internal bodily fluids. In freshwater, their bodies tend to absorb water and expel excess salts.

In freshwater a fish is hypertonic to its environment, it means fish is saltier and water continuously moves into the fishes body via Osmosis. When freshwater fish is placed in salt water the freshwater fish is hypotonic or less salty when compared to the environment. When a freshwater animal is placed in seawater, the higher concentration of salt in the seawater causes water to move out of the animal’s cells and tissues, leading to dehydration and osmotic stress. This means water continuously moves out of the fish body and lead to the death of the fish. Hence Many freshwater animals cannot live for long in sea water.

Conversely, marine animals are adapted to high-salinity environments and their bodies are equipped to expel excess salt and retain water. When placed in freshwater, water tends to enter their cells, causing them to swell and potentially burst, leading to osmotic imbalance and stress.

Thus, the inability to regulate osmotic pressure effectively in a different salinity environment makes it difficult for freshwater animals to survive in seawater and vice versa.