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.

Correct Answer: Option 3 low concentration→ high concentration (ATP used)

Active transport is the mediated process of moving particles across a biological membrane against a concentration gradient. Active transport uses energy, unlike passive transport, which does not use any energy. ATP, a form or chemical energy, is needed for this process to take place. Indeed, this setup is an active transport. Since it uses chemical energy, it is termed as primary active transport. If electrochemical gradient was used in the process, it is called secondary active transport.

Option 1 is incorrect because no energy is needed to move particles from high concentration to low concentration. Since there is no energy or ATP (adenosine triphosphate) needed for the process to happen, it is a passive transport.

Option 2 is incorrect. Just like option 1, no energy or ATP was used in the process therefore it is a passive transport.

Option 4 is incorrect. ATP is needed in order to transport particles from low to high concentration.