Which of the following topics is illegal for an employer to inquire about during an interview?

The topic of citizenship is illegal for an employer to inquire about during an interview because it can lead to discrimination based on national origin and is governed by immigration law. The law prohibits employers from asking about a candidate's citizenship status before hiring, as it is essential to focus on qualifications and abilities rather than personal characteristics that are irrelevant to job performance.

This question emphasizes the importance of understanding employment law and the protections afforded to candidates during the job application process. Other topics, like age, can be sensitive, but there are specific regulations that allow for limited inquiry if relevant to job functions, while questions about a criminal record or educational background are generally permissible and relevant, assuming they comply with applicable laws and regulations.