Final answer:
The yellow part of an NFPA diamond indicates the Instability Hazard of a substance, showing the potential for it to undergo a violent chemical change.
Step-by-step explanation:
The yellow portion of an NFPA diamond and HMLS label represents the Instability Hazard of a substance. This part of the NFPA 704 Hazard Identification System indicates how readily the substance might undergo detonation or a violent chemical change. The NFPA diamond rating scale, ranging from 0 to 4, evaluates the severity of the hazard, where 0 means no hazard and 4 indicates an extremely hazardous situation. Thus, for the question presented, the correct answer is C) Instability Hazard.