The correct option is D All of the above
The most highly organized lymphoid tissues are in the thymus and lymph nodes, which are well defined encapsulated organs. In the spleen, the lymphoid tissue is a cylinder of loosely organized cells surrounding small arteries. In the bone marrow this tissue is mixed with the blood forming cells. The most diffuse lymphoid tissue is found in the loose connective tissue spaces beneath epithelial membranes, such as those that line the gastrointestinal tract and the respiratory system