Those substances that start burning at ignition temperature in presence of oxygen is called combustible substance....so the correct answer is A.

**Combustible Substance: Wax**

**Introduction**
A combustible substance refers to a material that has the ability to catch fire and burn in the presence of oxygen. Among the given options, wax is the correct answer as it is a combustible substance. Let's explore why wax is considered combustible in detail.

**Properties of Wax**
Wax is a hydrocarbon compound that is derived from natural sources such as plants, animals, or petroleum. It consists of long-chain hydrocarbons, which gives it its characteristic solid state at room temperature. Some common types of wax include beeswax, paraffin wax, and soy wax.

**Combustion Process**
Combustion is a chemical reaction that occurs when a combustible substance reacts with oxygen, producing heat, light, and various combustion products such as water vapor and carbon dioxide. The combustion process involves three essential components: fuel (combustible substance), oxygen, and heat.

**Wax as a Fuel**
Wax, being a hydrocarbon compound, contains a significant amount of carbon and hydrogen atoms. These atoms serve as the fuel component in the combustion process. When wax is heated to its ignition temperature and exposed to an adequate supply of oxygen, it undergoes combustion.

**Ignition Temperature**
Ignition temperature refers to the minimum temperature at which a substance can ignite and sustain combustion. The ignition temperature of wax varies depending on its composition and purity. For instance, the ignition temperature of paraffin wax is around 370-400°C (698-752°F).

**Combustion Reaction of Wax**
The combustion reaction of wax can be represented by the following equation:

Wax + Oxygen → Carbon Dioxide + Water + Heat + Light

The hydrocarbons in wax combine with oxygen to produce carbon dioxide (CO2) and water vapor (H2O) as combustion products. The release of heat and light is a result of the exothermic nature of the combustion reaction.

**Conclusion**
In conclusion, wax is considered a combustible substance due to its ability to undergo combustion when exposed to an ignition source and sufficient oxygen supply. When wax burns, it releases heat, light, and combustion products such as carbon dioxide and water vapor. Understanding the combustibility of substances is crucial for safety purposes and handling flammable materials responsibly.