Which of the following hazards is indicated by the yellow quadrant in the NFPA 704 system?

Answer: C — Reactivity.

In the NFPA 704 system, the yellow quadrant specifically represents reactivity hazards associated with a chemical substance. This system is designed to provide an immediate visual cue regarding the potential dangers of materials. The colors and numbers in the NFPA label indicate the degree of health risks, fire hazards, reactivity, and special hazards. When identifying the yellow quadrant, the numbers it contains provide insight into how reactive the material is. A higher number in this quadrant indicates a greater potential for an adverse reaction under emergency conditions, such as explosive reactions or the potential for violent reactions when mixed with water or other chemicals. Understanding this system is vital for first responders and hazardous materials personnel to quickly assess risks and take appropriate safety measures.