What attribute is essential to define what happens when a form is submitted?

The appropriate attribute that determines what occurs when a form is submitted is the form action attribute. This attribute specifies the URL where the form data should be sent when the user submits the form. It effectively directs the web browser to the location of the server-side script or resource that will process the submitted data.

By defining the action attribute, a developer indicates to the browser where to send the information captured in the form fields. Without this attribute, the form submission would not know where to direct the data, leading to potential errors or a lack of response after submission.

While the form method attribute is also critical, as it dictates how the form data is sent (e.g., through GET or POST), the action attribute is the primary requirement for defining the destination of the form submission. The form ID and class attributes are more about styling and JavaScript manipulation rather than defining the submission behavior.