There are two types of reset with two different lifecycles: question and assessment.
The question reset button appears if you have multiple attempts at a question and you answer it incorrectly. Once your available attempts are exhausted or you answer the question correctly, the reset button disappears and the question is considered complete.
Assessment reset is controlled by the assessmentResults component retry button (as Barry has described). When the assessmentResults retry button is clicked it'll reset the completion of all of the questions in the assessment, allowing you to attempt the whole assessment again.