Which one of the following industrial materials can be detected using a magnet?

Ferrous metals are the correct choice because they contain iron, which is magnetic. When exposed to a magnet, ferrous metals will be attracted to it due to their iron content. This property makes ferrous metals easily detectable using a magnetic field, which is a fundamental principle utilized in various industrial processes and applications, such as sorting materials or in manufacturing settings.

Aluminum and copper, on the other hand, are non-ferrous metals; they do not contain significant amounts of iron and therefore do not exhibit magnetic properties. Consequently, they will not be attracted to a magnet, making them undetectable through this method. Plastic is a non-metallic material and lacks any magnetic properties as well. Thus, understanding the magnetic properties of different materials is crucial in industrial applications, specifically for the detection and separation processes.