Which of the following materials is most resistant to the penetration of ionizing radiation?

Lead is recognized as the material most resistant to the penetration of ionizing radiation due to its high density and atomic number. When discussing the attenuation of radiation, density plays a crucial role; denser materials possess a greater number of atoms within a given volume, which increases the likelihood of interactions between the radiation and the material. Lead, being one of the densest common materials, provides effective shielding against various forms of ionizing radiation, including gamma rays and X-rays.

The effectiveness of lead as a radiation barrier is further supported by its high atomic number, which contributes to a higher probability of photon interactions through processes such as photoelectric absorption and Compton scattering. This makes lead particularly useful in medical and dental radiography environments where shielding is necessary to protect both practitioners and patients.

In contrast, materials like aluminum and copper, while still providing some degree of radiation shielding, do not offer the same level of protection as lead. Plastic, being a less dense and lower atomic number material, is even less effective against ionizing radiation. Thus, among the materials listed, lead stands out as the clear choice for optimal resistance to radiation penetration.