Who discovered radioactivity?
A.Ernest Rutherford
B. Neils Bohr
C.Albert Einstein
D. Henry Becquerel

Hint: The SI unit of radioactivity is named after this person. This person was also the first to observe phosphorescence in cathode ray tubes, which is treated as the first evidence of radioactivity. He was also awarded the Nobel prize in Physics in 1903 along with Marie and Pierre Curie.

Complete answer:
Radioactivity can be described as the process by which an unstable nucleus spontaneously breaks down resulting in the release of energy and matter from the nucleus. This happens when the binding energy of the nucleus isn’t sufficient to hold its nucleons together.
Radioactivity is said to be a stochastic process, which means that it is a random process since it is impossible to determine when an atom will decay, irrespective of for how long the atom has existed.
The decaying nucleus is called the parent radionuclide which produces at least one daughter nuclide.
The three main radioactive processes are alpha decay (helium nucleus production), beta decay (electron or positron production) and gamma decay (photon production).
Becquerel’s discovery was a case of an unplanned fortunate discovery while working with phosphorescent materials (that glow in the dark after exposure to light). He suspected that a glow produced in cathode ray tubes by x-rays might be associated with phosphorescence. He wrapped a photographic plate with black paper and placed various phosphorescent salts on it. When he used uranium salts, they caused a blackening of the plate which meant that there was some form of invisible radiation that could pass through the paper and cause the plate to react as if it was exposed to light. These rays were called “Becquerel Rays”. This evidence was later analysed theoretically by many other scientists but experiments by Ernest Rutherford classified them as alpha, beta and gamma rays based on their penetration ability.

The correct choice would thus be: D. Henri Becquerel.

Note:
- Remember that in gamma decay there is not necessarily any nuclear transmutation resulting in a completely new daughter nuclide. The energy of an excited nucleus is emitted as a gamma ray or photon, and the nucleus just comes back to the ground state.
- In terms of the discoveries, Ernest Rutherford along with Thomas Royds proved that alpha radiation is indeed helium nuclei, and later even beta rays. However, the first person to discover evidence of radioactivity is still said to be Henri Becquerel, while many others including Rutherford contributed to building on it with subsequent experimental progress.