Question

Oxygen is the waste product of:

photosynthesis
respiration
the nitrogen cycle
burning fossil fuels

Answer 1

The answer is photosynthesis (also been called carbon dioxide) oxygen is just a waste product when plants do photosynthesis.
Hope this help man

Answer 2

Its photosynthesis because Plants can do two important things: Use energy from the sun to turn CO2 (carbon dioxide) and H2O (water)