Final answer:
After beta decay, a Proton remains in the nucleus of the atom, as a neutron is converted into a proton while emitting a beta particle (an electron).
Step-by-step explanation:
The atom that remains after the beta decay described in the passage is a Proton (a). During beta decay, a neutron within the nucleus of an atom is transformed into a proton and emits a beta particle, which is a high-energy electron. This process results in the increase of the atomic number by one because the proton remains in the nucleus. The mass number does not change because a neutron has been replaced by a proton, which has approximately the same mass. The ejected electron is known as a beta particle, and the simultaneous emission of an antineutrino, which has negligible mass, is generally ignored in this context.