Answer: They attentively listen in class to the teacher.
Explanation:
Verbs must agree with their subjects in number. A singular subject requires a singular verb, and a plural subject requires a plural verb.
If it were She attentively listen in class to the teacher, the subject and verb would not agree. It has to be She attentively listens in class to the teacher.
Example:
1. A hiker (walk, walks) along trails that are not steep.