Which of the following types of ionizing radiation has the most mass?

The type of ionizing radiation with the most mass is alpha radiation. Alpha particles are composed of two protons and two neutrons, making them relatively heavy as compared to other forms of radiation. This composition gives alpha particles a significant mass, which is approximately four times that of a hydrogen atom.

In contrast, beta radiation consists of electrons or positrons, which have a much smaller mass, effectively making them nearly massless in terms of atomic physics. Gamma radiation, which is a form of electromagnetic radiation, has no mass at all and travels at the speed of light. Neutrons, while they do have mass, are not a common form of radiation emitted in radioactive decay and do not fall under the same category as the other common types like alpha and beta. Thus, when considering the types of radiation presented, alpha particles emerge as the heaviest due to their substantial nucleonic structure.