Answer: The night
Explanation:
Remember that a plant does two things, it either produces energy by photosynthesis, in which case it takes up the carbon dioxide and releases oxygen, or consumes energy to grow/carry out the functions of life.
In the second case, the plant takes the glucose, or sugar, that it made during photosynthesis and breaks it into pieces, releasing energy. This step is accompanied by the release of carbon dioxide into the atmosphere. This happens all the time, but since the plant does not produce sugar and release oxygen during the night, the night is when it produces more CO2 than O2.
