The correct option is (a).
Explanation
Materials are classified based on their interaction with magnetic fields. Magnetic materials (specifically ferromagnetic materials) are those that are strongly attracted to magnets and can be magnetized themselves. Non-magnetic materials do not exhibit significant attraction to magnets.
Analysis of Materials:
- 1. Iron (Correct): Iron is a ferromagnetic material. It possesses magnetic domains that align with an external magnetic field, causing strong attraction.
- 2. Nickel (Correct): Nickel is a ferromagnetic element. Like iron, it is strongly attracted to magnets and is used in various magnetic alloys.
- 3. Cobalt (Correct): Cobalt is a ferromagnetic metal. It is widely used in the production of high-strength permanent magnets.
- 4. Plastic (Incorrect): Plastic is a polymer and qualifies as a non-magnetic material. It does not respond visibly to magnetic fields and cannot be magnetized.
Key Takeaway:
Iron, Nickel, and Cobalt are the classic examples of ferromagnetic materials, which are strongly attracted to magnets. Most organic polymers, such as plastics, are non-magnetic.