When a participant chooses not to answer a question and selects "Submit", a prompt will notify the participant of any unanswered questions and suggest for them to add input:
Please note, the survey can be submitted without answering the non-required questions. This notification is only suggesting the participant go back and offer input. Our surveys are designed with this feature in mind in an attempt to maximize the quality and quantity of engagement. To require engagement on a question, you'll need to adjust the settings on the question.
See also: