Answer:
1. Lena, who sees new viewpoints on a topic.
4. Adi, who better recalls what he has learned.
5. Yuto, who understands lessons better.
6. Riko, who looks forward to learning more.
Explanation:
From taking information from lessons viewed in class, to having suggestion for additional topics to study for, the four of these students demonstrate that they can yield some sort of contribution to a study group.
In the case of Yuron, letting the others do all the work is essentially a non-existent contribution to the group, and regarding Roni, not focusing their whole attention in the task at hand would make for a poor performance during the study session.