The correct option is A NO CHANGE
A phrasal verb is a verb combined with an adverb or preposition to
provide a meaning different from the literal meaning of that individual
verb. The underlined phrase in the given sentence is a phrasal verb of the form verb (fell) + adverb (apart). It means 'broke into pieces or fragments', referring to the vase that broke when it fell, hence it's the correct phrase and no change is necessary, so option A is correct. 'Fell down' is a phrasal verb that means 'to fail, be unsuccessful' or simply refers to the act of falling down; it is not required in the sentence as it's already mentioned that the vase fell on the floor, hence option B is incorrect. 'Fell together' means 'became identical, became one' like in 'as we found the last piece of the puzzle, the entire story fell together in place', it does not refer to the breaking of the vase. Hence option C is incorrect. 'Fall apart' is the present tense form of the underlined phrase, whereas the given sentence is in the past tense, proven by the verb 'threw' which is also in the past tense. Hence option D is incorrect.