Oxygen gas is not a water pollutant; it is essential for the survival of most aquatic life (fish, microorganisms, etc.) through the process of cellular respiration.

A. Warm water is a water pollutant because it decreases the amount of dissolved oxygen that can be held in the water and can stress aquatic organisms adapted to cooler temperatures.

B. inorganic fertilizers: are major water pollutants because the nitrates and phosphates they contain lead to eutrophication, a process where excessive algal growth depletes oxygen and harms other aquatic life.

D. biodegradable waste: large amounts of this waste in water are pollutants because their decomposition by microorganisms consumes significant amounts of dissolved oxygen, leading to oxygen-depletion pollution and the potential death of fish and other aquatic organisms.

:. The correct answer is oxygen gas - option C