Final answer:
T cells develop in the thymus and B cells develop in the bone marrow, making option 4) thymus; bone marrow the correct answer to the question about where T-cell and B-cell development occurs in mammals.
Step-by-step explanation:
In mammals, T-cell development occurs in the thymus, while B-cell development occurs predominantly in the bone marrow. When it comes to immune cell development, all lymphocytes are produced in the bone marrow. Specifically, B cells complete their maturation in the bone marrow, which is why they are named B cells ('B' standing for 'bone'). On the other hand, progenitor cells that will become T cells migrate to the thymus, where they mature into functional T cells, hence the term T cells ('T' for 'thymus').
The thymus is a central organ in shaping the T-cell repertoire and involves a selection process that eliminates T cells with receptors that could lead to autoimmunity. After completing their maturation, these cells leave the thymus and join other immune cells to perform their function in the adaptive immune response. In contrast, B cells, after maturation in the bone marrow, enter the bloodstream and lymphatic systems to search for pathogens.
Given this information, the correct answer to the student's question is: T-cell development occurs in the thymus, while B-cell development occurs predominantly in the bone marrow. Therefore, the answer to the question is option 4) thymus; bone marrow.