Explanation:

- Alexander Fleming (Option 3):

- Discovered penicillin in 1928.

- Noticed mold killing bacteria in a petri dish.

- Led to the development of antibiotics.

- Correct Answer

- Robert Koch (Option 1):

- Known for identifying the cause of tuberculosis, cholera, and anthrax.

- Developed Koch's postulates for linking pathogens to disease.

- Ernst Chain (Option 2):

- Helped develop and mass-produce penicillin with Howard Florey.

- Worked on purifying the antibiotic.

- Louis Pasteur (Option 4):

- Made breakthroughs in vaccination and pasteurization.

- Known for contributions to germ theory.