When a healthy freshwater fish is placed in saltwater, it will face a challenge called osmoregulation. Osmoregulation is the process by which an organism maintains the balance of water and salts in its body. Freshwater fish have specialized bodies that are adapted to live in a freshwater environment, where the water has a lower salt concentration than their bodies.

When a freshwater fish is placed in saltwater, the higher salt concentration in the water causes water molecules to move out of the fish`s body through osmosis. This leads to dehydration, as the fish loses water faster than it can replace it. As a result, the fish`s cells lose water, and it cannot function properly. If not addressed, this dehydration can ultimately lead to the death of the fish.

Option 1 correctly explains that a healthy freshwater fish placed in saltwater becomes dehydrated and dies. This dehydration is a result of the fish`s inability to regulate the exchange of water and salts with its surrounding environment.