Which of the following materials poses a flammability risk?

Aerosol spray cans pose a significant flammability risk due to the pressurized contents and the flammable propellants often used within them. When aerosol cans are exposed to heat or opened near a flame, they can explode or ignite, creating dangerous fire hazards. This characteristic makes them particularly hazardous in environments where open flames or high temperatures are present.

While other materials listed, such as paper products and wooden pallets, also pose flammability risks due to their combustible nature, aerosol spray cans are particularly concerning due to their potential for rapid ignition and explosion. Glass containers, on the other hand, generally do not pose a flammability risk, as they do not burn.