Which type of radiation is characterized as the most massive and least penetrating?

Alpha particles are composed of two protons and two neutrons, making them relatively heavy compared to other types of radiation such as beta particles, gamma rays, and X-rays. This substantial mass is a critical characteristic that distinguishes alpha particles from the lighter and more penetrating forms of radiation.

Because of their mass, alpha particles have a very limited ability to penetrate materials. They can be blocked by a sheet of paper or even the outer layer of human skin, which means they don't travel far and cannot penetrate deeply into substances. This physical property makes them the least penetrating type of radiation among the options provided.

In contrast, beta particles are lighter and can penetrate more than alpha particles but are still somewhat limited. Gamma rays and X-rays, being forms of electromagnetic radiation with no mass, are highly penetrating and can pass through most materials, requiring dense shielding like lead or thick concrete to effectively block them.

Thus, the defining characteristics of alpha particles—being the most massive and least penetrating—make them the correct answer to the question regarding the type of radiation described.