Marie Curie is renowned for her groundbreaking work on Radioactivity, making it the correct answer. Along with her husband Pierre Curie, she conducted pioneering research that led to the discovery of the elements polonium and radium. While Henri Becquerel initially observed radioactivity, the Curies extensively investigated and named the phenomenon, revolutionizing our understanding of atomic structure and energy. Her contributions earned her two Nobel Prizes, in Physics and Chemistry.
The other options are incorrect. Penicillin was discovered by Alexander Fleming. The principles of Electromagnetism were developed by numerous scientists, including Michael Faraday and James Clerk Maxwell. The Theory of Relativity was formulated by Albert Einstein. Marie Curie's legacy is firmly rooted in her profound discoveries in the field of radioactivity.