Fire tube boilers are a type of boiler commonly used in industrial applications. They are designed to transfer heat from hot gases produced by combustion to the water in the boiler. Fire tube boilers are categorized based on their method of firing, which can be either internally or externally.

1. Internally fired:
- Fire tube boilers that are internally fired have their combustion chamber located inside the boiler shell.
- The fuel, such as coal or gas, is burned inside the furnace, generating hot gases.
- These hot gases then travel through the tubes, transferring heat to the surrounding water.
- The hot gases finally exit through the chimney or stack.
- This design allows for a compact boiler size and efficient heat transfer.

2. Externally fired:
- Fire tube boilers that are externally fired have their combustion chamber located outside the boiler shell.
- The fuel is burned in a separate furnace or firebox, and the hot gases generated are then passed through the tubes.
- These hot gases transfer heat to the water surrounding the tubes.
- This design is commonly used in locomotive boilers, where the firebox is separate from the boiler shell.
- Externally fired boilers are generally larger in size compared to internally fired boilers.

3. Internally as well as externally fired:
- Some fire tube boilers can be designed to have both internal and external firing.
- In such cases, there are two separate combustion chambers: one inside the boiler shell and one outside.
- This configuration provides flexibility in fuel choices and combustion options.
- For example, an internally fired furnace can be used for solid fuel combustion, while an external furnace can be used for oil or gas firing.

4. None of these:
- This option is incorrect as fire tube boilers can be either internally or externally fired, or a combination of both.

In conclusion, the correct answer is option 'A', which states that fire tube boilers are internally fired. They have a combustion chamber located inside the boiler shell, where the fuel is burned to generate hot gases that transfer heat to the water through the tubes.