Finish the terrified babysitter's text message to her best friend.

The front door is locked from the inside, so the pizza delivery guy _____ the one making those spooky noises upstairs!

The correct answer is couldn't have been.

We use couldn't have (or can't have) + past participle to deduce that a past event was impossible. Because the door is locked from the inside, the babysitter logically deduces that the delivery guy cannot possibly be inside the house.

Must have been would mean she is certain he is upstairs, which contradicts the locked door.