Final answer:
A rational consumer will engage in an activity if the expected marginal benefit is greater than the expected marginal cost.
Step-by-step explanation:
A rational consumer will engage in an activity if the expected marginal benefit is greater than the expected marginal cost. This means that the benefits they anticipate receiving from the activity should outweigh the costs they expect to incur. For example, if someone is considering whether to buy a new car, they will only do so if they believe the benefits of owning the car (such as convenience and transportation) outweigh the costs (such as the price of the car, insurance, and maintenance).