Here's why:

* Photosynthesis is the process where plants use sunlight, water, and carbon dioxide to create their own food (sugar) and release oxygen as a byproduct.

Let's break down the roles of the other molecules:

* Oxygen is a product of photosynthesis, not an ingredient.

* Water is an ingredient used in photosynthesis.

* Nitrogen is an essential nutrient for plants but is not directly involved in the process of photosynthesis.