The correct answer is option 2, Landsteiner. Blood groups were discovered by Karl Landsteiner, an Austrian biologist and physician, in the early 20th century. Landsteiner conducted extensive research on blood transfusion and observed that when blood from different individuals was mixed, clumping or agglutination occurred. He identified and classified these agglutinogens (now known as antigens) into different blood groups - A, B, AB, and O. Landsteiner`s discovery revolutionized the field of medicine, as it allowed for safe and successful blood transfusions. His findings earned him the Nobel Prize in Physiology or Medicine in 1930. It is important to note that options 1 (Altmann), 3 (Losch), and 4 (Ronald Ross) are incorrect, as they are not associated with the discovery of blood groups.