In adults. RBC is formed in

The correct option is "the bone marrow of long bones".

To understand why this is correct, we can break down the process of red blood cell (RBC) formation (erythropoiesis) and how it changes throughout human development:
1. Embryonic stage: In the very early stages of embryonic development, RBC formation first occurs in the yolk sac.
2. Fetal stage: As the fetus develops, the liver and the spleen take over as the primary sites of RBC production.
3. Adult stage: In adults, the primary site of erythropoiesis shifts completely. The red bone marrow, which is found in the cavities of long bones (such as the femur and humerus) as well as flat bones (like the sternum, ribs, and pelvis), becomes the sole site of normal red blood cell production.

Let's briefly examine why the other options are incorrect for adults:
- The liver: While it is a primary site of RBC production during fetal life, in healthy adults, it no longer forms RBCs (though it helps break down old ones).
- The thymus: This organ is primarily involved in the maturation of T-lymphocytes (white blood cells of the immune system) and does not produce RBCs.
- The spleen: Similar to the liver, it acts as a site for RBC production in the fetus, but in adults, it serves to filter blood and destroy worn-out RBCs rather than forming new ones.

Therefore, in adults, red blood cells are formed in the bone marrow of long bones.