Fast process load changes can be challenging to handle, as they can lead to significant disturbances in the system. In order to effectively control the process and maintain stability, a suitable controller is required. Among the given options, the PD controller is the most appropriate choice for handling fast process load changes.

Below are the reasons why a PD controller is the best option:

1. Proportional Control:
- The PD controller consists of a proportional term (P), which provides control action based on the current error between the desired setpoint and the measured process variable.
- The proportional control helps in reducing the steady-state error and provides an initial corrective action based on the magnitude of the error.

2. Derivative Control:
- The derivative term (D) in the PD controller provides control action based on the rate of change of the error.
- In the case of fast process load changes, the rate of change of the error can be high. The derivative control helps in capturing this rate of change and provides additional corrective action accordingly.
- By considering the rate of change, the derivative control helps in predicting the future behavior of the process variable and responds quickly to any sudden changes.

3. Handling Fast Load Changes:
- Fast process load changes can lead to large deviations in the process variable, which can result in instability and poor control performance.
- The PD controller with its proportional and derivative terms is effective in handling fast load changes.
- The proportional term provides immediate control action based on the current error, while the derivative term helps in capturing the rapid changes and reacting accordingly.
- The combination of proportional and derivative control enables the PD controller to quickly respond to fast load changes and stabilize the process variable.

In summary, the PD controller is the most suitable option for handling fast process load changes due to its ability to provide immediate control action based on the current error (proportional control) and respond to rapid changes in the error (derivative control).