What attribute is added to input elements to require user input in a form?

The attribute that is added to input elements to require user input in a form is "required." This attribute is part of the HTML5 specification and is used to ensure that the user cannot submit the form without filling out the designated input field. When the "required" attribute is present on an input element, the browser performs a validation check before allowing the form to be submitted. If the field is left empty, the browser will typically display a message prompting the user to provide the necessary input. This functionality improves user experience by preventing incomplete form submissions and enhancing data integrity.

The other terms, while related to the idea of needing to provide information, do not correspond to a recognized HTML attribute that enforces this requirement. "Validated," "mandatory," and "obligatory" do not have the same defined function in HTML, making "required" the appropriate choice for ensuring user input in forms.