What he wants to hear
The most important part of your answer is this: you should sound at peace. Your date wants to see that you don’t cringe, or look at the ceiling, or change the subject when he asks you this. He wants to see that the question doesn’t evoke fear, nervousness or depression because that means you either have a ton of baggage, or aren’t totally over a breakup yet. He also wants to hear that you’re A) Not a serial monogamist and actually take time to yourself between relationships but B) Have actually been in a serious relationship or two, so he knows you’re datable.