The correct option is (d) Discovery of the process of producing monoclonal antibodies
To explain I would say: The B-cell cancer myeloma was known, researchers understood that these cancerous B-cells all produce a single type of antibody. This theory was used to study the structure of antibodies. Later the process of producing monoclonal antibodies invented by Georges Köhler and César Milstein shared the Nobel Prize in Physiology or Medicine in 1984 for the discovery.