Answer:
The correct answer would be b)regulation.
In general, regulation is term used when something is being controlled or adjusted.
Similarly, in plants stomatas are used to regulate the amount of oxygen and carbon dioxide in the plant.
Generally, when water is abundant in plant and sunlight is available, stomatas get open and allow the uptake of carbon dioxide from the environment. Carbon dioxide is an essential element required for photosynthesis.
In addition, it releases oxygen (byproduct of photosynthesis) out of the plant. The excess of oxygen in the plant can inhibit the growth of the plant as it competitively binds with an enzyme RuBisCO.