The correct answer is option 'C', which is 4.

Explanation:
CST-9 sleepers are a type of railway sleepers used in the construction of railway tracks. These sleepers are made of reinforced concrete and have a unique design that includes cotters. Cotters are small pieces of metal used to secure the rail to the sleeper.

The number of cotters used in CST-9 sleepers is 4. These cotters are strategically placed to ensure the rail is securely fastened to the sleeper, providing stability and support to the track.

The use of cotters in CST-9 sleepers is crucial for the proper functioning of the railway track. They help in preventing the rail from shifting or moving, thereby reducing the risk of accidents and ensuring smooth and safe train operations.

Cotters are typically inserted into holes drilled in the sleeper and then tightened using a cotter pin. The pin is inserted through the hole in the cotter and secured in place, ensuring a tight and secure connection between the rail and the sleeper.

By using 4 cotters in CST-9 sleepers, the rail is firmly held in position, preventing any lateral movement or displacement. This ensures that the track remains stable and safe for train operations.

In conclusion, the correct number of cotters used in CST-9 sleepers is 4. These cotters play a vital role in securing the rail to the sleeper, providing stability to the track and ensuring safe train operations.