There are 2 areas where you can configure "Compulsory" items:
This configuration applies a whole question.
Just below the Question area, check Is this a compulsory Question?. This forces the user to respond to the question at audit time. Although the user will be able to ignore the question, the user will not be able to move the audit status to "Completed" unless this question has been answered.
This configuration applies to an individual response to a question.
Click the edit button for any response to a question, and you will see several configuration options.
Check Is this a compulsory response. This prevents the user from progressing to the next question until a response has been selected.