Oxygen is the waste product of:

photosynthesis
respiration
the nitrogen cycle
burning fossil fuels

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

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

C) to ensure each new cell has an exact copy of the dna

True but only in the south coastal areas during september and early october.