Which type of material is specifically characterized by being a compressed gas?

The correct choice is flammable gas, as it is the only type among the given options that is explicitly characterized by being a compressed gas. Compressed gases often include those that can ignite and sustain combustion when mixed with air; therefore, they are classified as flammable gases.

Flammable gases typically exist in a compressed state within containers, making them easy to transport and store. When released, they can form explosive mixtures with air, which is a significant safety consideration in handling such materials. Their gaseous state at atmospheric pressure allows for rapid dispersion, which is crucial in risk assessment and safety management regarding their use.

On the other hand, solid explosives, liquid oxidizers, and corrosive liquids, while they might have their own hazards, do not specifically refer to materials that are compressed gases. Solid explosives exist in solid form and require specific conditions to detonate, liquid oxidizers are typically in liquid form and aid in combustion processes, and corrosive liquids can damage materials and tissues, but none of them is categorized as compressed gases.