Red bone marrow, thymus; sites where stem cells become immunocompetent
a. chyle
b. lacteal
c. lymph
d. primary lymphatic organs
e. secondary lymphatic organs

Primary lymphatic organs

Explanation:

Primary lymphoid organ is the site of formation and maturation of lymphocyte. These lymphoid organs provide proper environment for the maturation of immune cells.

The primary lymphoid organ are red bone marrow and thymus. B- cells born and mature in the bone marrow. T cells from the site of origin migrate to the thymus for the maturation.

Thus, the correct answer is option (D).

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".