The correct answer is Thymus.

- The thymus is a primary lymphoid organ that plays a crucial role in the development and maturation of T-lymphocytes (T-cells), which are essential for adaptive immunity.
- The most distinctive feature of the thymus is that it undergoes significant involution after puberty.
- This involution involves a gradual replacement of the thymic lymphoid tissue with adipose (fat) tissue, leading to a marked reduction in its size and activity.
- Although the thymus continues to function throughout life, its capacity to produce new T-cells declines considerably after puberty.

In contrast:
- The spleen remains relatively constant in size and function throughout life; it filters blood and participates in immune responses but does not undergo involution.
- The liver is a vital metabolic and detoxifying organ that does not involute after puberty.
- The pancreas also does not undergo involution; it continues to perform its exocrine and endocrine functions throughout life.

Reference: Robbins Basic Pathology, 10th Edition, Chapter 3: Inflammation and Repair, Page 87