Concept:The correct phrasal verb is “fallen out,” which means to disagree or quarrel with someone.Explanation:The sentence requires an idiomatic phrase showing a disagreement between the teacher and the principal.“Fallen out with” is the standard expression for having a quarrel.“Fallen down” means to collapse physically, so it is incorrect here.“Fallen off” is wrongly written, and “fallen apart” means to break into pieces or lose emotional control.Thus, the completed sentence is: “The teacher has fallen out with the principal.”Answer:D. fallen out