When do Event Handler actions react?

Event Handler actions are primarily designed to respond to specific triggers that occur within the application. The correct choice, which states that they react when there is a change in input parameters value, reflects how these actions listen for changes that may affect the application's behavior, data flow, or UI updates.

In the context of OutSystems and mobile development, input parameters commonly signify values that are passed to screens or actions, and any alteration in these values can necessitate certain processes—like data refreshing, condition checks, or activating additional logic. An event handler linked to input parameters ensures that your application can dynamically adapt to user interactions and data changes.

The other scenarios mentioned, such as pressing a button, loading the application, or logging in, are indeed triggers that can initiate event handling, but these triggers don't specifically pertain to changes in input parameter values, which is the focus of the correct answer.