Head developed by a centrifugal pump depends on various factors, including the impeller diameter and speed, as well as the type of casing. Let's discuss each of these factors in detail.

1. Impeller diameter:
The impeller is a rotating component of the centrifugal pump that imparts energy to the fluid and increases its pressure. The impeller diameter plays a crucial role in determining the head developed by the pump. A larger impeller diameter results in a higher head, as it allows for greater fluid flow and imparts more energy to the fluid. Therefore, increasing the impeller diameter increases the head developed by the pump.

2. Speed:
The rotational speed of the impeller directly affects the head developed by the pump. The head developed is proportional to the square of the impeller speed. This means that increasing the speed of the impeller results in a significant increase in the head developed. However, it is important to note that there is an optimum speed for each pump, beyond which the efficiency of the pump decreases. Operating the pump at this optimum speed ensures maximum head development and efficiency.

3. Type of casing:
The casing of a centrifugal pump is the stationary part that surrounds the impeller and guides the fluid flow. The type of casing used in the pump can have an impact on the head developed. Different casing designs have varying levels of efficiency and can affect the fluid flow patterns within the pump. The casing design should be optimized to minimize losses and maximize head development. Therefore, the type of casing used in the pump can influence the head developed.

4. A and B above:
The correct answer is option 'D' because both the impeller diameter and speed play significant roles in determining the head developed by a centrifugal pump. Increasing either the impeller diameter or speed, or both, will result in a higher head developed. Additionally, the type of casing used can also affect the head developed, making it another important factor to consider.

In conclusion, the head developed by a centrifugal pump is influenced by the impeller diameter, speed, and type of casing. Increasing the impeller diameter and speed will generally result in a higher head developed, while the type of casing used can impact the pump's efficiency and fluid flow patterns. It is essential to optimize these factors to achieve the desired performance from the centrifugal pump.