Which of the following substances is classified as a flammable gas?

The classification of flammable gases is based on their physical state and behavior at standard atmospheric conditions. A substance is considered flammable gas if it exists in a gaseous state at normal temperature and pressure and can ignite when exposed to an ignition source. This characteristic is crucial for safety protocols in laboratories, particularly when dealing with open flames or sparks.

The definition provided in the correct answer encompasses all gaseous substances under normal conditions, affirming their classification as flammable if they can support combustion. Understanding that flammable gases can easily form combustible mixtures with air is essential for laboratory safety and risk management.

In contrast, other choices describe different categories of materials. A liquid with a high flash point is less likely to ignite at room temperature and is thus not classified as flammable under normal conditions. A solid that burns persistently when ignited refers to combustible solids, which behave differently from gases. Lastly, a material that can explode when heated pertains to substances that may undergo explosive reactions rather than the continuous combustion characteristic of gases. Each of these distinctions is vital for recognizing and handling various hazardous substances safely.