The draught in locomotive boilers is produced by a steam jet. This is the correct answer because steam jets are commonly used in locomotive boilers to create a draught or draft, which is the flow of air through the boiler and chimney. The draught is necessary to maintain combustion and ensure efficient operation of the boiler.

Here is a detailed explanation of how a steam jet produces draught in locomotive boilers:

1. Purpose of draught in locomotive boilers:
- The primary purpose of draught in locomotive boilers is to supply the necessary oxygen for combustion and remove the combustion products (such as flue gases) from the boiler.
- It also helps in maintaining the required pressure inside the boiler and ensures the proper flow of gases through the chimney.

2. Steam jet as a method of producing draught:
- In a locomotive boiler, a steam jet is used to create the draught.
- The steam jet is created by injecting high-pressure steam into a nozzle or diffuser.
- As the steam expands through the nozzle, it creates a high-velocity jet of steam.
- This high-velocity steam jet creates a low-pressure area around it, which in turn creates a pressure difference between the inside and outside of the boiler.
- This pressure difference creates a flow of air through the boiler and chimney, thereby producing the draught.

3. Advantages of using a steam jet:
- Steam jets are simple and reliable devices for creating draught in locomotive boilers.
- They do not require any moving parts or external power sources, making them easy to maintain and operate.
- Steam jets can provide a strong draught, ensuring efficient combustion and better boiler performance.

In conclusion, the draught in locomotive boilers is produced by a steam jet. The steam jet creates a low-pressure area that induces the flow of air through the boiler and chimney, maintaining combustion and ensuring efficient operation of the boiler.