Final answer:
The most severe hazard category in terms of harm to human health is Extreme Hazard.
Step-by-step explanation:
The student has asked which hazard category is the most severe in terms of harm to human health. In the context of environmental health risks, the options provided are Moderate Hazard, High Hazard, Extreme Hazard, and Severe Hazard. The term "Extreme Hazard" generally indicates the highest level of risk and therefore can be considered the most severe category when it comes to potential harm to human health.