the world's population is one of the biggest _______(challenge) of the world添什么呢

the world's population is one of the biggest _______(challenge) of the world添什么呢
the world's population is one of the biggest _______(challenge) of the world
添什么呢

the world's population is one of the biggest _______(challenge) of the world添什么呢
challenges
这里的结构是one of + 形容词最高级 + 名词复数,意思是最……之一,所以用challenge的复数challenges.

challenges
one of+ 最高级+名词复数，表示最什么什么的之一