In which scenario would you likely use an event handler?

Using an event handler is essential when you want to execute a specific piece of code in response to particular user interactions, such as clicking a button, hovering over an element, or submitting a form. Event handlers allow the developer to define how the application should respond to these events, thus creating a dynamic and interactive user experience. For example, when a user clicks a button, an event handler can trigger a function that displays a message or updates part of the web page. This ability to respond in real-time to user actions is what makes event handlers a fundamental concept in programming for interactive applications.

The other scenarios described do not involve responding to user interactions in real-time. Defining a variable in a function is part of establishing logic and functionality within the code but does not relate to user-triggered events. Establishing a database connection pertains to backend operations rather than user interface events. Optimizing file storage is focused on data management and efficiency rather than interactivity with the user.