Answer: a. dissolved O2
Oxygen is a vital gas that is required by all living organisms for their respiration and cellular metabolism. It is an important abiotic (non-living factor) . The oxygen is abundantly present in the atmosphere which is easily respired by the land plants and animals but in the aquatic ecosystem the oxygen is dissolved from the atmosphere. The oxygen should be replenished as it is used by the aquatic plants and animals in the water.