Answer: The correct answer for the blank is- Photosynthesis.
Photosynthesis is a process of formation of food ( that is glucose, a carbohydrate) by green plants and algae.
In this process, green pigment ( chlorophyll) absorbs energy from sunlight in presence of water and carbon dioxide ( which is present in the atmosphere), and convert it into chemical energy ( ATP and NADPH), which is used in the formation of glucose. Oxygen is released in the surrounding ( by splitting of water molecule) in this process.
Thus, plants use the process of photosynthesis to remove carbon dioxide from the air.