Final answer:
The illegal question in a job interview in the U.S. is "Which state were you born in?" as it discriminates based on national origin, violating the Civil Rights Act of 1964.
Step-by-step explanation:
The question which asks, "Which state were you born in?" is illegal to ask in a job interview in the United States. This question potentially discriminates against a person based on their national origin, which is protected under the Civil Rights Act of 1964. Employers are required to avoid questions that relate to personal information not directly related to the job qualifications and duties. Instead, employers may inquire about an applicant's ability to legally work in the U.S. without asking about their birthplace directly.