Final answer:
The theory that asserts that motivation is essentially a decision about how much effort to exert to get what you want in a particular situation is Expectancy theory.
Step-by-step explanation:
The theory that asserts that motivation is essentially a decision about how much effort to exert to get what you want in a particular situation is Expectancy theory. According to this theory, people's motivation to act in a certain way is determined by their belief that their efforts will lead to specific outcomes or rewards. For example, if a student believes that studying hard will result in good grades, they will be motivated to exert more effort in their studies.