Which of the following is a visible indicator of combustion?

• A. Smoke
• B. Temperature rise
• C. Oxygen depletion
• D. Pressure increase

Answer: (A)

Smoke is the visible sign that combustion is occurring. When fuel burns, heat causes fuel fragments and gases to break down and release particulates, aerosols, and water vapor, which form the visible plume you can see. The color and density of that smoke can also give clues about how complete the burn is and what’s fueling it, but the key point is that smoke is something you can observe with the naked eye during combustion.

Temperature rise, while a real effect of burning, isn’t seen directly without tools or touch. Oxygen depletion happens as the fire consumes air, but you don’t observe it visually on its own. Pressure increase is an internal or mechanical change that might show up as noises, door or vent movement, or structural signs, not as a direct visual cue of burning.