Why do fishes die when taken out of water?

Concept: The respiratory system of a fish allows it to spend its entire life in water. Fishes do not need to come to the surface of the water very often to breathe air. It obtains the required amount of oxygen from water rather than air, via tiny blood vessels spread across the surface area of its gills.

Option A is that fish breathe through their gills. The structure of the gills allows fish to breathe underwater. Because they cannot breathe through atmospheric oxygen, the fish die when removed from the water. Therefore, this option is correct.
Option B is that the gills are densely packed with blood capillaries and can readily absorb dissolved oxygen in the water. This option is also correct in terms of gill structure. As previously stated, gill structures help a fish survive underwater but cannot help a fish survive outside of the water. Therefore, this option is also correct.

Hence, the correct answer is option C.