Which group indicates that the chemical is combustible when exposed to air?

The correct response identifies Group C as indicating that a chemical is combustible when exposed to air. This classification signals that these substances can ignite and support combustion upon contact with air, presenting a significant safety hazard, especially in environments where they may be stored or handled.

Combustibility is a critical factor in managing ammunition and materials involved in this field. Understanding the properties of Group C chemicals helps personnel to take appropriate precautions, such as ensuring proper ventilation and storage conditions, to prevent accidental ignition and maintain safety protocols.

In contrast, groups A, B, and D may represent chemicals with different characteristics related to their behavior in the presence of air, such as stability or non-combustibility, but they do not specifically denote the combustibility characteristic detailed in the question. Recognizing this distinction is vital for professionals working with hazardous materials.