______ to take the course must be the top students in the grade.

those brave enough
这句可以看成是those who are brave enough 的省略式，brave enough to take the course用来修饰those，这句话再省略点就是xx must be the top students in the grade.

Those brave enough

=those who are brave enough

enough足够的 用来修饰时，放于形容词后面

those brave enough
那些足够勇敢的
enough在修饰形容词时放在形容词后面。是固定用法

those brave enough to take the course must be the top students in the grade.

Those brave enough
enough修饰形容词，副词后置；brave enough是词组，修饰those后置

those brave enough
那些有足够勇气参加这个课程的同学一定会成为这个年级的尖子生