Which of the following types of radiation has the smallest mass?

Beta radiation consists of electrons or positrons, which are subatomic particles with very small mass compared to other types of radiation. Specifically, the mass of a beta particle (an electron) is approximately 1/1836 of that of a proton, making it significantly lighter than the particles associated with other types of radiation.

Alpha radiation, on the other hand, consists of helium nuclei (two protons and two neutrons), which have a much larger mass. Gamma radiation is not a particle but rather high-energy electromagnetic radiation, which can be considered massless in the context of particle mass. Neutron radiation consists of neutrons, which are fairly heavy subatomic particles as they have a mass comparable to that of protons.

By understanding the masses of these different types of radiation, it becomes evident that beta radiation is the lightest, thus establishing its correct identification in this context.