The correct answer is option 'B', which states that the fuel used for welding metals is a mixture of ethyne and oxygen. Let's understand why this is the correct answer.

1. Introduction to welding fuel:
- Welding is a process that involves joining two or more metal pieces together by melting and fusing them.
- To achieve this, a fuel is required to produce a high-temperature flame that can melt the metals.
- The fuel used for welding is typically a mixture of a combustible gas and oxygen.

2. Ethyne as a fuel:
- Ethyne, also known as acetylene, is a versatile and commonly used fuel in welding.
- It is a hydrocarbon gas that is highly flammable and can burn in the presence of oxygen.
- Ethyne has a high heat of combustion, making it suitable for providing the required temperature for welding.

3. Role of oxygen in welding:
- Oxygen is an essential component in the welding process as it supports combustion and enhances the heat of the flame.
- When oxygen is combined with the fuel gas, it increases the flame temperature and facilitates the melting of metals.
- The oxygen reacts with the fuel gas during combustion, resulting in the generation of a high-temperature flame.

4. Advantages of ethyne and oxygen mixture:
- The mixture of ethyne and oxygen offers several advantages for welding:
- High flame temperature: The combination of ethyne and oxygen produces a flame with a high temperature, reaching up to 3300°C. This temperature is sufficient for melting various metals.
- Controlled heat: The flame produced by ethyne and oxygen can be controlled easily, allowing welders to adjust the heat as needed for different metal types and thicknesses.
- Clean combustion: Ethyne and oxygen combustion produce a clean flame without excessive smoke or soot, ensuring better visibility during the welding process.

5. Other options:
- Option 'A' (Ethyne and hydrogen) is incorrect because hydrogen is not commonly used as a fuel for welding due to its high reactivity and explosive nature.
- Option 'C' (Ethyne and carbon dioxide) is incorrect as carbon dioxide is not used as a fuel in welding. It is often used as a shielding gas to protect the weld pool from atmospheric contamination.
- Option 'D' (Ethyne and nitrogen) is incorrect as nitrogen is an inert gas and does not support combustion, making it unsuitable as a fuel for welding.

In conclusion, the correct fuel mixture used for welding metals is a combination of ethyne and oxygen. This mixture provides a high-temperature flame necessary for melting metals and offers advantages such as controlled heat and clean combustion.

In gas welding ethyne is flammable and oxygen increases the flammability of ethyne. Thus these two mixture is used for gas welding.