The correct answer is option 3, Henri Becquerel. Henri Becquerel is credited with the discovery of radioactivity. In 1896, he accidentally discovered this phenomenon while conducting an experiment involving minerals containing uranium salts. He noticed that these minerals had the ability to produce invisible rays that could penetrate through materials and expose photographic plates, even in the absence of an external source of energy.

Although options 1 and 2 (Marie Curie and Pierre Curie) are closely associated with the study of radioactivity and made significant contributions to this field, they were not the ones who initially discovered it. Marie Curie and Pierre Curie conducted extensive research on radioactivity and made crucial discoveries, such as the isolation of radium and polonium, for which Marie Curie was awarded the Nobel Prize in Physics in 1903.

Option 4 (J. J. Thomson) is not related to the discovery of radioactivity. J. J. Thomson is known for his work on the discovery and identification of the electron and his contributions to the field of atomic structure.

Therefore, the correct answer is option 3.