ANS:A - flash point.

In the context of liquid petrofuels, momentary combustion is observed at its flash point. Here's an explanation:

• Flash Point: The flash point of a liquid petrofuel is the lowest temperature at which it produces enough vapor near its surface to form a flammable mixture with air that momentarily ignites or "flashes" when a flame is introduced under specified test conditions.
• Observation: At the flash point, the fuel emits vapors that can momentarily ignite but do not sustain combustion. This ignition is momentary and does not lead to sustained burning of the fuel.
• Significance: The flash point is a critical safety parameter for liquid petrofuels, as it indicates the temperature at which the fuel can pose a fire hazard under specific conditions. It is used in industry and transportation to classify and handle fuels safely.
• Comparison with Fire Point: The fire point of a fuel is the temperature at which it produces enough vapor to sustain combustion for at least 5 seconds after ignition. It is typically higher than the flash point.