If you require the customer to submit an answer - you must add the question to the 'Required Answers' list.
Open Routing and you'll spot a rule called 'Required Answers'
Open the rule and drag in the the question you require them to answer in the IF section. Add additional questions and combine with Logic (AND OR).
Please note: Hidden questions (set via other Routing Rules) don't become a requirement until they are shown.
Press Save when complete.