What type of radiation is most effectively blocked by paper?

The correct answer is that paper effectively blocks alpha particles. Alpha particles are relatively heavy and carry a positive charge, making them more likely to interact with and be stopped by materials that are not dense. Due to their larger mass and size compared to other forms of radiation, such as beta particles and gamma rays, alpha particles can be significantly attenuated by even lightweight materials like paper.

In contrast, beta particles, which are lighter and can penetrate further than alpha particles, require denser materials for effective shielding. Gamma rays, being highly penetrating electromagnetic radiation, require heavier and denser materials like lead for effective blocking. X-rays, similar to gamma rays, also have considerable penetration capabilities and would not be significantly stopped by paper. Thus, in terms of relative effectiveness in blocking radiation, paper is particularly efficient against alpha particles.