Answer: A). dissolved oxygen
Dissolved oxygen is the abiotic factor. Abiotic factors are the non-living components of the environment which can have an influence on the living organisms. Abiotic factors include soil, water, oxygen, temperature etc.
The underwater organism population gets oxygen from aquatic plants which produce oxygen by the process of photosynthesis. Therefore, oxygen is available in the dissolved state in water called as dissolved oxygen. Respiration by underwater animals and plants , decomposition and other chemical reactions requires oxygen. Utilization of dissolved oxygen by underwater animals can limit the overall concentration of dissolved oxygen which can affect the vital metabolism of underwater animals and therefore can limit their population.