Which rule is used to automatically set a property value when an input value changes?

The correct choice, which is the Declare Expression, is utilized to automatically set a property value when there’s a change in an input value. This rule type acts in real-time to evaluate conditions based on the input and updates the property accordingly whenever the specified input changes. This makes it particularly valuable for dynamic calculations or when a property's value needs to reflect the most current data without requiring additional user actions.

Declare Expressions are especially fitting for situations where you want to maintain data consistency across various parts of an application, as it automatically recalibrates the value based on dependencies defined in the expression itself. This allows for an efficiently managed data state with minimal manual intervention, enhancing user experience.

In contrast, the other options serve different purposes: Activities are used for procedural logic and executing steps in a predefined order, Data Transforms are typically employed to map data between different classes or transform data structures, while Decision Tables help in decision-making processes based on multiple conditions but do not automatically update property values in the same dynamic manner as Declare Expressions do.