All the lights in the house are flashing on and off and then the narrator says he "hears a creepy howling sound", the house then gets on flames and one of the cracks in the house then spits open and the whole house falls into the moat.
According to the options stated, the one which is correct is the last one: “The crack in the house splits open and the house falls into the moat”. The House of Usher, which can be assumed that is some house of somewhere in Europe in the 1800s, splits apart and collapsed by wiping away the remnants of the ancient family. Besides, Poe used a metaphor to resemble the fall of the House itself as an old decaying entity in itself.
