Which of the following sets includes the primary lymphoid organs?

Bone marrow and thymus

 The correct answer is (b) Bone marrow and thymus.

Primary lymphoid organs are the sites where lymphocytes originate and mature. These organs play a crucial role in the development and maturation of lymphocytes before they enter circulation.

The bone marrow is a primary lymphoid organ where B-cells develop and mature. It is responsible for producing a diverse population of B-cells that can recognize a wide range of antigens.

The thymus is another primary lymphoid organ where T-cells  mature. Immature T-cells, called thymocytes, undergo maturation and selection processes within the thymus, ensuring that they can recognize foreign antigens while not attacking the body's own tissues.

Lymph nodes, spleen, Peyer's patches, liver, and tonsils are examples of secondary lymphoid organs. These organs are involved in the activation and proliferation of mature lymphocytes, as well as the initiation of immune responses, by presenting antigens to lymphocytes.

Therefore, the primary lymphoid organs are the bone marrow and thymus, making option (b) the correct choice.