You should answer all of the questions truthfully. If you pass the interview, they will conduct a background investigation. If during their investigation, they discover that you were less than truthful about the smallest of things, you will be disqualified. You are applying for a position in which you have to uphold the law. Therefore, they are looking for integrity.
Most people are a little nervous during a job interview.
This is a normal response. To help you relax, pause before answering a question. Take just a brief moment to think about your answer before responding. This will help you to collect your thoughts. As you answer the questions, speak clearly and loudly.