Final answer:
Plasma cells are located in the medullary cords of a lymph node, which contain B cells and plasma cells that are crucial for the immune response.
Step-by-step explanation:
Plasma cells are found in the medullary cords of a lymph node. Lymph nodes are structured to filter lymph fluid and capture pathogens and antigens through various regions, including the germinal centers and medullary areas. The medullary cords contain B cells and plasma cells, which are B cells that have differentiated and now have the ability to secrete antibodies, key components of the immune system. Thus, the correct answer to where plasma cells are found in a lymph node is b. In the medullary cords.