The correct answer is option 3: Polyethylene.
Polyethylene is a type of polymer that is commonly used for making bullet-proof materials due to its high strength and impact resistance. It is a strong and durable polymer that absorbs the energy from a bullet, preventing it from penetrating the material.
Option 1: Polyvinyl chloride (PVC) is a versatile polymer that is used for various applications such as pipes, cables, and flooring. However, it is not typically used for making bullet-proof materials.
Option 2: Polystyrene is a lightweight polymer that is commonly used for packaging and insulation. It is not suitable for making bullet-proof materials as it lacks the necessary strength and impact resistance.
Option 4: Polyamide is a strong and rigid polymer that is commonly used for applications such as textiles and engineering plastics. While it has some impact resistance, it is not the preferred polymer for making bullet-proof materials.
In conclusion, the correct polymer used for making bullet-proof materials is polyethylene (option 3).