Answer: Expectancy theory
Step-by-step explanation:
The theory that suggests that the student will only work as hard as necessary to earn a "C" grade is the expectancy theory.
Expectancy theory states that an individual will act in a particular way due to the fact of what they believe will be the result of the behavior that they selected and thereby will select a particular behavior above orhers.
Here, the student simply selects his work above the school grade because he believes that a C is enough for him. Therefore, the answer will be expectancy theory.