You can set your questions as mandatory in the survey editor. This means that, when presented with a mandatory question, the respondent cannot continue until the question is answered.
To make a question mandatory:
1. Go to the 'Surveys' tab
2. Open an existing survey or create a new one
3. Click on an existing question or add a new one
4. Click 'Advanced options'
5. Click 'Make mandatory'
6. Remember to save your changes