To produce ionizing radiation, an atom must

The correct answer, which states that to produce ionizing radiation, an atom must add an electron, is based on the nature of ionizing radiation itself. Ionizing radiation occurs when an atom becomes unstable, which can happen through various processes including the addition of energy or particles in specific interactions.

In the case of adding an electron, while it can initially seem counterintuitive because electrons are typically negatively charged and do not contribute directly to ionizing radiation, one of the common mechanisms of ionization involves gaining energy that can lead to the ejection of electrons from the atom. When an electron is added, it can impact the atom's stability if the electron is associated with high-energy processes, such as during photon absorption.

It’s essential to recognize that the production of ionizing radiation often involves changes that lead to the emission of particles or electromagnetic radiation. The process of ionization involves typically expelling one or more electrons from the outer shell of an atom, rendering it positively charged and unstable, thus producing ionizing radiation.

The other options do not lead to the production of ionizing radiation directly and do not fit within the accepted understanding of how atoms interact when undergoing radioactive decay or ionization processes. For instance, adding a proton would increase the atomic number