Elodea, a type of aquatic plant, produces oxygen when exposed to light.

Here's why:

* Photosynthesis: Elodea, like most plants, performs photosynthesis. This process uses sunlight, carbon dioxide, and water to produce glucose (a sugar) for energy and oxygen as a byproduct.

* Oxygen Release: The oxygen produced during photosynthesis is released into the surrounding water, which is why you can often see bubbles forming around Elodea when it's in sunlight.

So, the answer is oxygen.