Answer : 2. "Thymus"
Explanation :

1. It is at its greatest in children.

2. With puberty, it eventually shrinks and is replaced by fat.

3. It produces and secretes thymosin, an essential hormone for T cell development and production.

4. Thymus gland is a small organ located under the breast bone in the upper chest.

5. It produces white blood cells, called lymphocytes, which protect the body from infection.