Final answer:
The blue diamond on an NFPA 704 symbol represents the level of health hazard a substance poses, with ratings from 0 (no hazard) to 4 (severe hazard).
Step-by-step explanation:
The NFPA 704 Hazard Identification System is a standardized label designed by the National Fire Protection Agency (NFPA) to indicate the hazards of chemicals. Within this system, the blue field on the hazard diamond symbol represents the level of health hazard that a substance poses. Therefore, the answer to the student's question is a) Health Hazard. This blue diamond evaluates the immediate or chronic health risks that exposure to the substance could entail, with ratings on a scale of 0 to 4, where 0 indicates no health hazard, and 4 indicates a severe hazard that could cause death or serious injury upon very short exposure.