Aquatic animals use_____dissolved in water to respire.
A. \[C{{O}_{2}}\]
B. \[{{H}_{2}}\]
C. \[{{O}_{2}}\]
D. \[{{H}_{2}}{{O}_{2}}\]

Hint: Aquatic animals like fish respire through extract oxygen dissolved in water with the help of specially designed organs called gills or directly through their skin.
Animals according to their natural habitat are classified as aquatic(water)animals and terrestrial (land)animals.

Complete step by step solution:
As all Aquatic animals have four chambered hearts therefore they respire through their specially designed Gills. They respire through extract oxygen dissolved in water. Some aquatic animals like frogs respire directly through their skins. Aquatic animals have scales and their body is streamlined so that they can freely move in water without any problem.
Gills are kind of feathery organs which are full of blood vessels. A fish breathes by taking water through their mouth and passing it out with the help of their gills. As water passes through the wall of the gills they extract dissolved oxygen from them and passes it to the brain cells .
Most aquatic animals like fish ,crabs, mollusks, crabs, prawns first need to extract oxygen from the surrounding water so that they can further go for cellular respiration. The main step for respiration is primary diffusion. At maximum there is 15 milligram of oxygen present in per liter of water. Hence fishes require the dissolved \[{{O}_{2}}\] that is present in water, to respirate.

Note: Some aquatic animals like dolphins and whales are mammals like humans. They give birth to babies instead of laying eggs so they have lungs like humans rather than gills so they breathe through their lungs with the help of their nose also known as a blowhole present on the top of their body surfaces.