Human Anatomy

Step 1: Lymphocytes, including natural killer (NK) cells, do arise from bone marrow and are present in blood and lymph — Assertion is true.
Step 2: Some lymphocytes migrate to the thymus to develop into mature T-cells — Reason is true.
Step 3: However, the Reason explains T-cell development, not the role of lymphocytes as NK cells — so option (B) is correct.

The thymus is a lobed organ located near the heart and beneath the breastbone (sternum) in the mediastinum. It plays a key role in the immune system by aiding T-cell development. The liver and spleen are in the abdomen, and the lungs, while near the heart, are not a single lobed organ beneath the breastbone.