The correct option is A Lymphoid tissue
The tissue, which forms the basic structure of lymphoid organs, spleen etc., is lymphoid tissue. Cells and organs that make up the lymphatic system, such as white blood cells (leukocytes), bone marrow, and the thymus, spleen, and lymph nodes. Lymphoid tissue has several different structural organizations related to its particular function in the immune response. The most highly organized lymphoid tissues are in the thymus and lymph nodes, which are well-defined encapsulated organs with easily identifiable architectures. In the spleen, the lymphoid tissue is a cylinder of loosely organized cells surrounding small arteries.