Answer: A. an electron
Beta particles are electrons or positrons (electrons with positive electric charge or antielectrons). Beta decay is a type of radioactive decay in which a beta ray is emitted from an atomic nucleus.
Beta decay occurs when, in an unstable nucleus with too many protons or too many neutrons, one of the protons or neutrons transforms into the other. In beta minus decay, a neutron is broken down into a proton, an electron, and an antineutrino (the neutrino antiparticle, meaning it has an opposite charge to the neutrino). In beta decay plus, a prototype breaks down into a neutron, a positron and a neutrino.
