Final answer:
The type of Activity class used for user interaction is a User task, which involves human input to complete a process step and is distinct from automated tasks and process initiators.
Step-by-step explanation:
In the context of user interface design, particularly within the realm of software engineering or process modeling, the type of Activity class used for user interaction is b) User task. This is a task where human input is required to complete a step in a process. The user task is intended for activities that cannot be automated and thus require human participation. Examples include filling out a form, approving a document, or performing a physical check.
The other options provided, such as script task or start event, serve different purposes within a workflow or process model. A script task is generally used for automated, script-based actions, whereas a start event signifies the beginning of a process.