It's a tricky sentence.
"Etre à la porte de quelqu'un" means "to be at the door of someone" or "to be near to someone/something" in most of the case.
But "Etre à la porte" may be an expression too which means "locked out" and to go further "Mettre quelqu'un à la porte" means "to fire" or "to exclude".
So "I am locked out of my house", "I am at the door of my house" and "I am near to my house" should be correct.
This is a misleading exercise. "I am locked out of my house" is not an exact translation and I don't know if I would've gotten it right if I hadn't had it be multiple choice. Duolingo should warn users that it is a saying and not an exact translation to not confuse them. Having spoken French my whole life and having lived in France for 5 years, I have not once heard someone use this phrase to say they are locked out of their house.