The correct answer is: A. Electron
In beta decay, a neutron in the nucleus of an atom decays into a proton, an electron, and an antineutrino. The electron is emitted from the nucleus, while the proton and antineutrino remain in the nucleus.
Beta decay is a type of radioactive decay, which is a process in which an unstable atomic nucleus loses energy by radiation. The energy released in beta decay is carried away by the electron and the antineutrino.
Beta decay is a common process in nature, and it is responsible for the production of many of the elements that are found in the universe. It is also used in some medical treatments, such as radiation therapy.
The other options are incorrect because they do not represent the only particle that is emitted in beta decay. A proton is not emitted in beta decay, and a neutron is only emitted in beta decay if it decays into a proton, an electron, and an antineutrino.