The correct answer is stomata.
CAM plants they close their stomata during the day and open at night.
CAM is also known as Crassulacean acid metabolism. CAM is termed as a pathway for carbon fixation which evolves in some plants as an adaptation to conditions which are arid.
If a plant is using CAM in full then the stomata which is found in leaves remain closed during the day reason being it reduces evapotranspiration and at night it opens to collect carbon dioxide.
This carbon dioxide is being stored in vacuoles as four carbon acid malate at night whereby the malate is being transported to chloroplasts and converted back to carbon dioxide which is known to be used in the process of photosynthesis.