The correct answer is: B. an atomic pile.
An atomic pile, also known as a nuclear reactor, is a device that uses nuclear fission to generate heat and electricity. Nuclear fission is a process in which a heavy nucleus, such as uranium-235, splits into two smaller nuclei, releasing a large amount of energy. This energy can be used to heat water, which then turns into steam to drive a turbine. The turbine generates electricity.
Radioactive substances are produced in atomic piles when the uranium-235 nuclei split. The two smaller nuclei that are produced are radioactive, and they decay into other radioactive elements. This process of radioactive decay continues until the radioactive elements reach a stable state.
The other options are incorrect because they are not devices that produce radioactive substances. A transistor is a semiconductor device that is used to amplify and switch electronic signals. An electron gun is a device that emits a beam of electrons. A Wilson cloud chamber is a device that is used to detect and study ionizing radiation.