The green structure that uses energy from the sun to produce food for a plant is called chloroplast.

Here's why:

* Chloroplasts are organelles found within plant cells.

* They contain a green pigment called chlorophyll, which absorbs sunlight.

* This absorbed light energy is used in a process called photosynthesis, where the plant converts carbon dioxide and water into glucose (sugar) and oxygen.

* The glucose is the plant's food source, providing energy for growth and other life processes.