Concept: Phrasal verbs with “fall” have different meanings based on the particle used.Explanation:“Fell down” means to drop to the ground.“Fell on” means to attack or to be directed toward something.“Fell through” means a plan or arrangement failed to happen.“Fell off” means to become detached or to decrease.Since the sentence indicates that something planned did not take place, the correct phrasal verb is “fell through.”Answer: The correct option is C. fell through.