Question

It was his parents help———— enabled him to buy an apartment in the big city.

求问这道英语为什么选B，不选D

Answer 1

It was his parents help that enabled him to buy an apartment in the big city. 是他的父母帮助他在大城市里买了一套公寓。
这是一个强调句，it is that强调句子的谓语。
如果用who的话，it代替parents不合适。

Answer 2

1.这是强调句型
2.若是定语从句先行词是help 也不能用
who的