He discovered X-Rays.

Joseph Lister pioneered antiseptic surgical techniques based on germ theory - he had no role in discovering X-rays.

Alexander Fleming is credited with discovering penicillin in 1928, not X-rays.

Robert Koch identified the bacterial causes of diseases like tuberculosis and anthrax, unrelated to the discovery of X-ray radiation.

Wilhelm Conrad Roentgen discovered X-rays in 1895 while experimenting with cathode ray tubes, observing that a new form of radiation could penetrate solid objects and produce images on photographic plates. His discovery revolutionized medical diagnostics and earned him the first Nobel Prize in Physics in 1901.