If nearly all organisms break down food by the process of cellular respiration, why doesn't the earth run out of oxygen? where does the co2 waste go?

Plants actually use CO2 in photosynthesis during the day, as they need sunlight to do photosynthesis, and they do cellular respiration at night to create the energy that they will need the next day. Thus yes CO2 would buildup in the night, but is used by plants in the day. As a result of photosynthesis, oxygen is made, so that the organisms will have oxygen needed for cellular respiration.

Your answer is b. codominant