Correct Answer: Option 3 Guard cells change the size of leaf openings, regulating the exchange of gases.

The guard cells regulate the stomata, which are microscopic holes in a plant leaf which allows gases to enter and leave and water vapor to leave as well. The ability of the guard cell to close during periods of limited water availability for the plant allows the plant to maintain water homeostasis. This is an example of feedback mechanism in plants. A feedback mechanism occurs when the level of one substance influences the level of another substance or activity of another organ.   Option 4 states about respiration, which is not a feedback mechanism of plants.

Option 2 is incorrect. Oxygen is a product of photosynthesis. Therefore, if there is decreased rate of oxygen, less oxygen will be also produced by the chloroplast. Carbon dioxide, not nitrogen, is used up by the chloroplast during photosynthesis. Thus, option 1 is incorrect.