The correct answer is A) flammable gas. Butadiene, when inhibited, is classified as a flammable gas. This is based on the regulations and criteria set forth by the US Coast Guard for the transport and handling of hazardous materials. Flammable gases are defined as materials that are gases at 20°C (68°F) or less and have a flammable range when mixed with air. Butadiene meets these criteria, which is why it is labeled as a flammable gas. The other options are incorrect because combustible liquids have a higher flashpoint, flammable liquids have a lower flashpoint, and flammable solids do not meet the criteria for a gas.