Concept:The correct phrase is an idiomatic phrasal verb that means to have a disagreement or quarrel.
Explanation:The sentence needs a phrasal verb that can be followed by "with" and expresses a dispute between two people.
"Fallen out" is the correct idiomatic expression for having an argument and ceasing to be friendly.
"Fallen apart" means to break into pieces or fail, which does not fit the context.
"Fallen of" is grammatically incorrect because "of" is not used after "fallen" in this sense.
"Fallen down" means to collapse or drop, which is also unsuitable here.
Therefore, the sentence should read: "The teacher has fallen out with the principal."
Answer:B. fallen out