Answer:
Teachers say that only grades motivate students.
Step-by-step explanation:
This is the best option, because it states that the only thing that motivates students are grades, and it is said by the teachers, who are responsible for the motivation and the grades of the students, they are together on a daily basis, so they must know something about what motivates them.
"Teachers say... it states a common belief among teachers... than only grades, and this is the students only motivation."