What are the tiny openings on the underside of a leaf that allow for gas exchange?

The tiny openings on the underside of a leaf that facilitate gas exchange are known as stomata. These structures are essential for the process of photosynthesis, as they allow carbon dioxide to enter the leaf while enabling oxygen and water vapor to exit. Stomata open and close to regulate the exchange of gases and to help prevent water loss through transpiration.

In relation to the context of the question, chloroplasts are the organelles where photosynthesis occurs, but are not openings for gas exchange. Guard cells are specialized cells that surround each stoma and control its opening and closing, playing a crucial role in regulating gas exchange and water loss, but they themselves are not the openings. Vascular bundles consist of xylem and phloem, which transport water, nutrients, and food throughout the plant, but they do not function as openings for gas exchange. Thus, stomata is the correct choice in this scenario as they are directly responsible for the gas exchange in leaves.