If you would like to make sure a question is answered in your survey then you can mark it as “Required”. The participant will not be able to complete the survey until all required questions are answered.
In the Survey Editor, if you click on the “Modify” icon on the question you want to make required, it will open the “Question Settings”. In the “Options” section you will see a toggle switch next to “Required question”. If you turn the toggle on then the question will become mandatory, indicated by the asterisk (*) symbol at the end of the question text.