At, but you also have to add the word "the". I think I heard the doorbell. There's somebody at the door.

At the door. In the door would mean they are stuck inside the door haha. You could say in the doorWAY , though! On the door would be sitting on top of the door. Ouch!! That sounds painful. 🤣

at is correct

“At the” door. “The” needs to be added before the word door.

Welcome :-)