Constant choke type carburettors maintain a constant airflow through the carburettor regardless of the engine's operating conditions. This means that the choke size remains the same regardless of the throttle position or engine load. Among the given options, the correct answer is option 'D' - S.U. carburettor. Let's discuss each option in detail to understand why the S.U. carburettor is the only constant choke type carburettor among the given options.

Zenith carburettor:
- Zenith carburettor is a variable choke type carburettor.
- It has a movable choke plate that controls the amount of air entering the carburettor based on the throttle position.
- As the throttle opens, the choke plate also opens, allowing more air to enter the carburettor.
- This design allows for better fuel-air mixture control based on the engine's operating conditions.

Soiex carburettor:
- Soiex carburettor is a variable choke type carburettor.
- It also has a movable choke plate that adjusts the amount of air entering the carburettor based on the throttle position.
- The choke plate opens as the throttle opens, allowing more air to enter the carburettor.
- This design provides better fuel-air mixture control for improved engine performance.

Carter carburettor:
- Carter carburettor is a variable choke type carburettor.
- It uses a variable venturi design to control the airflow into the carburettor.
- As the throttle opens, the venturi size changes, allowing more air to enter the carburettor.
- This design provides better fuel-air mixture control for optimal engine performance.

S.U. carburettor:
- S.U. carburettor is a constant choke type carburettor.
- It has a fixed choke size that remains the same regardless of the throttle position or engine load.
- Instead of a movable choke plate or variable venturi, the S.U. carburettor uses a piston to control the fuel-air mixture.
- The piston's position is adjusted by the engine's suction, which allows the right amount of fuel to mix with the incoming air.

In conclusion, among the given options, the S.U. carburettor is the only constant choke type carburettor. The other options - Zenith, Soiex, and Carter carburettors - are all variable choke type carburettors that adjust the airflow based on the throttle position or venturi size.