Answer:
Oxygen molecules
Step-by-step explanation:
Plants, during the process of photosynthesis, give off oxygen molecules as a by-product of the process.
Plants, through the process of photosynthesis, use the energy gotten from sunlight to produce glucose from carbon dioxide and water molecules, and in the process, oxygen molecules are released into the atmosphere as by-product.
Therefore, the label A in the diagram represents molecules of water that is given off by plants.