Answer:
The theory that suggests the student will work only as hard as necessary to earn a C grade is: expectancy theory.
Step-by-step explanation:
Expectancy Theory is related to motivation and behavior. According to it, people choose a certain behavior instead of others only when motivated by the outcome brought by that behavior. Applying it to the scenario we are analyzing here: the student does not see any advantages in working hard to achieve grades higher than C. Higher grades will not change the outcome, so there is no reason to spend more time and energy to get them. Thus, he chooses the behavior that he feels motivated enough to perform.