Thermite is a mixture of iron oxide and aluminium powder. It is a highly exothermic reaction that produces molten iron and a large amount of heat. This reaction is often used in welding and metal cutting applications due to its high temperature and ability to melt metals.

- The correct answer is option 'D', aluminium powder. Let's understand why aluminium powder is the correct choice.

Explaining the reactants:
- Iron oxide: Iron oxide is a compound that consists of iron and oxygen atoms. It is commonly found in nature as rust, which is a form of iron oxide. In the context of thermite, iron oxide acts as the oxidizing agent, providing oxygen atoms for the reaction.

- Aluminium powder: Aluminium powder is a fine, flammable powder made up of small particles of aluminium. It is highly reactive and readily combines with oxygen, making it an excellent reducing agent in the thermite reaction. The high reactivity of aluminium allows it to react with the oxygen from the iron oxide, resulting in a vigorous exothermic reaction.

Understanding the reaction:
- The thermite reaction can be represented by the following equation:
2Al + Fe2O3 → Al2O3 + 2Fe

- In this reaction, aluminium (Al) reacts with iron(III) oxide (Fe2O3) to produce aluminium oxide (Al2O3) and molten iron (Fe). The reaction is highly exothermic and releases a large amount of heat.

Role of aluminium powder:
- Aluminium powder plays a crucial role in the thermite reaction. It acts as the reducing agent, which means it supplies electrons to the reactants and facilitates the transfer of oxygen atoms from the iron oxide to the aluminium. As a result, the aluminium is oxidized, and the iron oxide is reduced.

- The high reactivity of aluminium powder ensures a rapid and complete reaction with the iron oxide. The heat generated during the reaction is intense enough to melt the resulting molten iron, which can be used for various applications such as welding or metal cutting.

Conclusion:
- In summary, thermite is a mixture of iron oxide and aluminium powder. The aluminium powder serves as the reducing agent and reacts with the iron oxide to produce molten iron and release a significant amount of heat. This reaction is widely used in various industrial applications, including welding and metal cutting.