Question

He made ambitious plans but all of them fell ________

Answer 1

HEY MATE
$\huge\mathfrak\red{Solution}$

through

Answer 2

ANSWER:

He made ambitious plans but all of them fell apart/through.

EXPLANATION:

Here in this given sentence one needs to use correct grammar to complete the sentence and therefore apart or through are used.  

"Grammar is very important" in any sentence as it leads to convey the right meaning to the readers who are all reading it. It reflects the tense and the feel with which the writer is conveying it.