Which radiation has the least penetration power?

Alpha radiation has the least penetration power among the types of radiation listed. This is due to its composition and the nature of its interaction with matter. Alpha particles consist of two protons and two neutrons, making them relatively large and heavy compared to other forms of radiation. Because of their size, alpha particles have a very low ability to penetrate materials. They can be stopped by a sheet of paper or even the outer layer of human skin.

In contrast, beta particles, which are much smaller subatomic particles, can penetrate further, passing through paper but typically being stopped by plastic or a few millimeters of aluminum. Gamma radiation and X-rays are forms of electromagnetic radiation with much higher penetration abilities, capable of passing through thick layers of materials, including human tissue, which is why they require significant shielding, such as lead or several centimeters of concrete, to be effectively blocked.

Thus, the fundamental property of alpha radiation, being composed of heavy particles that interact strongly with matter, results in its limited penetration ability, confirming it as the correct answer in this context.