Answer:
magnets attract only some materials.
Step-by-step explanation:
Based on the investigations described, the students would most likely propose the theory that:
magnets attract only some materials.
The fact that iron nails were stuck to the bar magnet while other materials like plastic pieces, wood shavings, and paper pieces were not attracted suggests that magnets have selective attraction and do not attract all materials. This observation aligns with the theory that magnets have specific magnetic properties and only attract certain materials, primarily those containing iron or exhibiting ferromagnetism.