The correct answer is d. formal operational
The formal operational stage of cognitive development according to Piaget refers to the final stage of cognitive development. This stage usually occurs at age 12 until adulthood. The formal operational stage of cognitive development involves the ability to think abstractly, use logical reasoning, think creatively and come up with solutions. Since Bethany's solving a physics problem by developing and testing several hypotheses, she is in the formal operational stage of cognitive development.