What type of declarative rule would detect changes in properties on a form and trigger actions?

The selected answer denotes the type of declarative rule specifically designed to recognize when properties change within a form and subsequently initiate specified actions. Declare OnChange rules monitor designated properties for modifications and allow developers to define responses to those changes directly.

When a property that the rule is set to observe changes, actions such as recalculating values, displaying different sections of the form, or altering user interface elements can be executed automatically. This mechanism is crucial for creating dynamic and responsive applications that tailor the user experience based on real-time data input.

Declare Expressions are used for calculations rather than property change detection. Declare Triggers are focused on specific situations like saving data or sending notifications and do not directly relate to observing property changes. Declare Index is associated with performance optimization by indexing properties for quicker database searches, which does not involve property change detection. Hence, the nature of a Declare OnChange rule makes it the appropriate choice for the question asked.