Talk about your interest in the beat and how you have been following all the major news stories of that particular beat. Explain to the interviewer how you have the qualities required to excel in the beat. If you have any contacts in the beat, you can talk about it, but do not brag. Show any published work, in the relevant beat. If you are not sure which beat you want to work in, you can mention 2-3 and say you are open to working in either of them.