Make Questions/Fields Mandatory in a Questionnaire
First, navigate to the Knowledge Base ('Setup' >> 'Knowledge Base') and then click 'Questionnaire Template' in the 'Questionnaire Templates' section:
Next, click on the questionnaire template that you want to modify. Then click 'Subsections'
From there, select 'Fields' and then change the drop-down for 'Mandatory' to 'Yes' for any field that you require a response for:
Make sure that the field is checked (check-box on the left of the field name). Once you've made the changes make sure to save the questionnaire.