All his plans for starting his own business fell .

Đáp án B.
(a plan) fall through = fail
Loại A vì fall in (phr v): xếp hàng (lính).
Ex: The sergeant ordered his soldiers to fall in.
Loại C vì fall down (phr v): được chứng minh là không đúng.
Ex: When the government assigned an archaeologist to investigate the site, the theory that has been put forward finally fell down.
Loại D vì fall away (phr v): biến mất.
Ex: At first we doubted his excuse for turning up late but later his friend told us the story. Then, our doubts fell away.