Final answer:
The 'W with a slash' on an NFPA 704 symbol indicates that the chemical has a special hazard, particularly an unusual reaction with water.
Step-by-step explanation:
In the NFPA 704 Hazard Identification System, a W with a slash through it indicates a special hazard. Specifically, this symbol signifies that the chemical has an unusual reaction with water. The NFPA 704 diamond consists of four colored diamonds: red indicates flammability, blue health hazards, yellow reactivity, and white at the bottom points out special hazards. The special hazards section can include notes on substances being oxidizers, corrosive, or water reactive, among other unique risks. Each hazard is rated on a scale from 0 to 4, with 0 being no hazard and 4 indicating extreme hazard.
The NFPA 704 symbol is a hazard identification system that provides safety information about certain substances. The symbol consists of a diamond divided into four colored quadrants. The white (bottom) diamond indicates special hazards. It can include symbols such as a W with a slash through it (W) to indicate a specific special hazard. Therefore, the correct answer to your question is 4) Special hazard.