T-lymphocytes originate from:
A. Thymus
B. Bone marrow
C. Liver
D. None of the above

Hint: There are two lymphocytes- B and T lymphocytes. T-lymphocytes provide cell-mediated immunity. They become active only when they are exposed to antigens.

Complete answer:
The thymus is one of the primary lymphoid organs. Primary lymphoid organs are those that synthesize the lymphocytes and provide them with a suitable environment for differentiation and development. They are the thymus and the bone marrow. It is a superior mediastinal structure that regresses at puberty. T-lymphocytes are a part of the myeloid group of leukocytes that confer the body with cell-mediated immunity. They phagocytose the cells or they cover the antigens with molecules so that the B-lymphocytes and the other phagocytic molecules can neutralize them.
There are many types of T-lymphocytes:
1. Cytotoxic T cells: They kill the target cells by releasing cytotoxic granules into the cells that are to be killed. These cells recognize the specific antigen when the antigens are presented by MHC I (Major Histocompatibility Complex) molecules that are present on the surface of all the nucleated cells
2. Memory T cells: They are the antigen-specific and long-living cells that confer the phenomenon of secondary immunity to organisms
3. Helper T cells: They aid in the cytotoxic functions of the Tc cells.
The normal T cell count of a healthy person ranges between 500 and 1,600 cells per cubic millimeter of blood. The developing progenitors of the T-lymphocytes are present within the thymus. These cells are the thymocytes.
Hence, the correct answer is option A.

Note: The bone marrow is the site of production, development, and differentiation of B-lymphocytes. The appendix, tonsils, Peyer’s patches of the small intestine, lymph nodes, and the thymus are the secondary lymphoid organs. These are the organs wherein the inactive lymphocytes get activated on getting in contact with the antigens.