Answer: Option (A)
Explanation: Plant has the ability to produce food by absorbing carbon dioxide, and releasing oxygen, this process is known as the photosynthesis process. The CO₂ present in the atmosphere is produced by the humans and organisms during exhalation.
The more CO₂ a plant receives, the more it can undergo photosynthesis, and there is reduction in the level of CO₂.
Thus, according to the given condition, if we add plants to the flask, then the CO₂ content will reduce and will be provided with oxygen in return.
Thus, the correct answer is option (A).