Answer:
The correct answer is dissolved oxygen.
Explanation:
Oxygen is an essential gas, which is needed by all the living species for their cellular metabolism and respiration. It is an essential non-living element. The presence of oxygen in the atmosphere is in abundance, which makes it easy for the land plants and animals to respire, however, in the aquatic ecosystem, the oxygen is found in a dissolved state. The oxygen needs to get restored as it is utilized by the aquatic animals and plants in order to survive.
