The correct answer is option 2, Alexander Fleming. Alexander Fleming discovered penicillin, which is a type of antibiotic. He made this important discovery in 1928 while studying bacteria in his laboratory. Fleming noticed that a mold called Penicillium notatum was able to kill the bacteria he was working with. This observation led to the development of penicillin as a powerful and widely used medicine to treat bacterial infections. Lord Lister, option 1, is known for developing antiseptic surgical techniques, but he did not discover penicillin. Karl Landsteiner, option 3, discovered the blood groups, and Walter Reed, option 4, contributed to the research on yellow fever. So, the correct answer is indeed option 2, Alexander Fleming.