The correct option is D Thymocytes
B-cells (or B-lymphocytes) and T-cells (or T-lymphocytes) are the lymphocytes, that are mature and have receptors on their surface and thus are able to bind to antigens and help the foreign substance to be removed from the body.
Thymocytes are the developing T-cells of hematopoietic origin and are immature.
Leukocytes are the mature white blood cells.
Thus, the correct answer is the option (C), 'Thymocytes'.