Option D, taking turns looking at several people.
I'll go over why the other three options are bad ideas.
Option A and B do maintain eye contact, but it'll be inferred by the audience that you're looking at solely one person.
It's pretty obvious that Option C is a terrible idea, since the point of maintaining eye contact is to be looking at your audience.
The reason Option D is the best choice is because it keeps the audience engaged. By looking around a room, your audience will infer that you're addressing everyone in the room.
-T.B.
