Final answer:
In the NFPA 704 symbol, the number that represents the highest hazard in any category (flammability, health, or reactivity) is '4'. This scale ranges from 0 signifying no hazard to 4 indicating extreme hazard.
Step-by-step explanation:
The NFPA 704 symbol, also known as the "hazard diamond," is used by the National Fire Protection Agency to convey specific hazards associated with chemicals. On the NFPA 704 symbol, each type of hazard is categorized by a different color and rated on a scale from 0 (no hazard) to 4 (extremely hazardous). The highest hazard level represented on this scale is 4. This applies to all categories, including the flammability (red diamond), health (blue diamond), and reactivity (yellow diamond). The white (bottom) diamond offers additional information like whether a substance is an oxidizer, water-reactive, corrosive, or poses other special hazards.