The correct answer is option 4: Steel. Radioactive materials should be kept in containers made of steel for several reasons. Steel is a strong and durable material that can effectively contain the radioactive material and prevent leaks or accidents. It also provides shielding against radiation, as steel has high density which helps to absorb the radiation and protect the surrounding environment and individuals from exposure. Steel is also resistant to corrosion, which is important for maintaining the integrity and longevity of the container. Although options 1 (Fe), 2 (AL), and 3 (Pb) are commonly used materials, they are not as suitable as steel for containing radioactive materials due to their lower strength, shielding ability, or susceptibility to corrosion.