Which type of radiation has the smallest mass?

Gamma rays have the smallest mass of all the types of radiation listed. They are a form of electromagnetic radiation, similar to X-rays, and do not possess mass because they are packets of energy known as photons.

In contrast, alpha particles are composed of two protons and two neutrons, which gives them a significant mass compared to other forms of radiation. Beta particles, which are high-energy, high-speed electrons or positrons emitted by certain types of radioactive decay, have a small mass but are still more massive than photons. Electron emissions, also falling under the category of beta radiation, retain this small but measurable mass.

Therefore, among the options, gamma rays are indeed the lightest and do not have a mass in the conventional sense, making this option the correct choice when focusing on radiation types in terms of mass.