The correct answer to the given question is option b. if the marginal benefit of studying an extra hour exceeds its marginal cost.

According to the marginal analysis, an additional unit of a given activity (i.e. an extra hour of study in the given scenario) should be carried out only when the marginal benefit exceeds the marginal cost. If the marginal benefit is equal to or less than the marginal cost, the additional unit for the activity should be avoided.