Answer:
3. regulating gas exchange
Step-by-step explanation:
What the student is observing are the structures responsible for regulating gas exchange in vegetables. These structures are called stomata and allow the loss of water vapor (which leaves the plant into the atmosphere), and the entry and exit and carbon dioxide or oxygen, which as you may know are gases directly involved in photosynthesis.
Stomata are only opened (allowing small openings to be visualized in the plants with the help of a microscope), when the entry or exit of a gas is extremely important for the panta to maintain its homeostasis efficiently.