Alpha radiation can be blocked by which of the following?

Alpha radiation is composed of helium nuclei, which are relatively heavy and carry a positive charge. Due to their structure, alpha particles have low penetration power. They can be easily stopped by materials with minimal density.

The skin serves as a barrier to alpha radiation, effectively absorbing these particles and preventing them from penetrating into biological tissues. Since alpha particles do not have sufficient energy to breach the outer layer of the skin, this makes skin an effective protective medium against alpha radiation.

While clothing can also provide a certain level of protection against alpha particles, skin is a more direct and effective barrier. Options involving heavier materials like concrete, earth, or lead typically pertain to blocking beta or gamma radiation rather than alpha particles. Similarly, while oil and water can provide some shielding, they are not as effective as the skin. Therefore, the most straightforward and appropriate answer to blocking alpha radiation is skin.