That's:
D. Activate the bear trap underneath my Welcome mat.
Seriously,
the correct answer is to take a strong position of defensive cover in a position away from the door and to announce in a LOUD voice "Who are you and what do you want!", or if it is your bedroom door "I'm armed and the Police are on the way" or something similar. Even if I lived alone and it was my bedroom door, I'd still be very much disinclined to shoot through the door without knowing who I am shooting at.
The detail that the person normally sleeping next to you isn't there, in the above scenario, makes shooting through the door an extremely stupid if not criminal idea.
It is probably the main squeeze, sneaking back after a sweet rendezvous with Zee Lover
.
If it is your front door and you think that spying through the view hole is a good idea, well, all I can say is that people on the outside can often see that someone is on the other side peeking through, not to mention that I think there are lenses that allow someone to view into your house through the spy hole. Standing with only a door as cover between you and a potential murderer is a bad idea, cause after all, they might choose to ventilate you through the door as soon as they become aware that you are on the other side of the door.