**Core Concept**
Radioactivity is a phenomenon where unstable atomic nuclei lose energy by emitting radiation in the form of particles or electromagnetic waves. This process involves the decay of radioactive isotopes, resulting in the emission of radiation that can be detected and measured.

**Why the Correct Answer is Right**
Radioactivity was discovered by Henri Becquerel in 1896. He observed that uranium salts emitted a form of radiation that could penetrate solid objects and cause photographic plates to become exposed, even in the absence of light. This discovery led to a deeper understanding of the structure of atoms and the nature of radioactivity. The process involves the decay of unstable nuclei, where an atom loses energy by emitting alpha, beta, or gamma radiation.

**Why Each Wrong Option is Incorrect**
**Option A:** Marie Curie is often associated with radioactivity, but she discovered the elements polonium and radium, which are radioactive, and isolated them from pitchblende ore. This is not the discovery of radioactivity itself.
**Option B:** Wilhelm Conrad Röntgen discovered X-rays in 1895, which are a form of electromagnetic radiation, but he did not discover radioactivity.
**Option C:** Ernest Rutherford's work on radioactivity led to a deeper understanding of the structure of atoms, but he did not discover it.

**Clinical Pearl / High-Yield Fact**
The discovery of radioactivity led to the development of nuclear medicine, which uses radioactive isotopes for diagnostic and therapeutic purposes.

**Correct Answer: B. Wilhelm Conrad Röntgen.**