Which material is recognized as a good magnetic material?

Iron is recognized as a good magnetic material because it possesses a high permeability which allows it to be easily magnetized and to retain its magnetic properties. This quality makes iron an essential component in many electrical applications, such as transformers and electric motors, where magnetic fields are utilized to induce or control electrical currents.

In contrast, aluminum and copper, while they are excellent conductors of electricity, do not have the same magnetic properties as iron. Neither material can be magnetized to the degree that iron can, and they do not retain magnetism effectively. Plastic, being a non-metallic material, has no magnetic properties and cannot be magnetized at all. Thus, it is clear that iron stands out as the material that embodies the characteristics required for effective magnetism, making it the correct choice for this question.