Which one of the following radiations is least penetrating?

Alpha rays are indeed the least penetrating type of radiation. They consist of two protons and two neutrons, essentially making them helium nuclei. Due to their relatively large mass and charge, alpha particles have limited ability to penetrate materials. They can be stopped by a sheet of paper or even the outer layer of human skin.

In contrast, beta rays, gamma rays, and x-rays are all more penetrating forms of radiation. Beta rays, which are electrons or positrons, can penetrate materials more effectively than alpha particles but are still limited by materials like plastic or glass. Gamma rays and x-rays, being electromagnetic radiation, can penetrate even more deeply into materials, requiring dense substances like lead or several centimeters of concrete for effective shielding.

Understanding the characteristics of different types of radiation helps in assessing their potential health risks and the appropriate safety measures necessary when dealing with radioactive materials.