The correct answer is: a. in photosynthesizing cells in dark periods and in other tissues all the time
Cellular respiration is a process, more precisely set of metabolic reactions used to convert biochemical energy from nutrients into energy (ATP). One of the steps in cellular respiration is oxidation of pyruvate that occurs in mitochondria of all cells.
During the dark period only respiration takes place (oxygen is used, carbon dioxide is reeased). But, during daytime when the light is bright, photosynthesis in photosynthesizing cells produces oxygen and glucose faster than respiration consumes it.