Combustibility is the ability of a substance to burn or ignite when exposed to a flame or heat source. Among the given options, hydrogen is the only substance that is combustible. Let's explore why hydrogen is combustible and why the other options are not.

1. Hydrogen:
Hydrogen is a highly flammable gas and is considered combustible. It has a low ignition energy, meaning it can easily ignite and burn in the presence of a spark or flame. When hydrogen combines with oxygen (in the air), it forms an explosive mixture that can be easily ignited. This reaction is highly exothermic, releasing a large amount of heat energy.

2. Oxygen:
Oxygen is not combustible. It is a non-flammable gas that supports combustion. This means that oxygen is necessary for other substances to burn, but it does not burn itself. Oxygen supports combustion by reacting with combustible materials, such as fuels, to produce heat and light. It acts as an oxidizer, allowing other substances to burn more readily.

3. Carbon dioxide:
Carbon dioxide is not combustible. It is a non-flammable gas that does not support combustion. Carbon dioxide does not react with heat or flame, and it does not burn. In fact, carbon dioxide is often used as a fire extinguisher because it displaces oxygen and removes the necessary component for combustion.

4. All of these:
The correct answer is option 'C' - Hydrogen. Only hydrogen is combustible among the given options. Oxygen and carbon dioxide are non-combustible gases.