Final answer:
In the process of photosynthesis, oxygen is produced alongside carbohydrates such as glucose. Oxygen is considered a waste product in this process, but it is essential for the respiration of most living organisms. So, the correct option is 3) oxygen.
Step-by-step explanation:
In addition to carbohydrates, oxygen is the other substance that is a product of photosynthesis. During this process, photosynthetic organisms such as plants use sunlight to convert carbon dioxide and water into energy-storing glucose, a simple carbohydrate, and oxygen. The general chemical equation for photosynthesis is carbon dioxide + water + sunlight → glucose + oxygen gas. Here, glucose serves as a primary energy source for cells, while oxygen is released as a 'waste' product, although it is vital for the respiratory processes of most other living organisms.