Making a question required is not necessary if you only have one question, or if your survey has multiple questions that do not relate. If you are setting up logic to show a follow up question, based on an earlier question's response, it can be a good idea to make that question required to proceed.

A required question will show in your post-purchase survey with a * next to it.
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To make a question required:
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Navigate to your Survey editor
At the bottom of the question, you'll see Required in grey, click Required so it turns orange, click Save changes
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This will also show in the right side panel, under the heading 'General> Required' as checked
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