The students are busy ___(listen) to the teacher.

问题描述:

The students are busy ___(listen) to the teacher.

Be busy doing sth是一个固定句型,所以应该是
The students are busy listening to the teacher.

listening
be busy doing 固定搭配

The students are busy listening to the teacher.
学生们正忙着听老师(讲课)
be busy doing sth,是固定词组,意思是忙于做某事