Final answer:
The core change in the formal operational stage is abstract thinking and hypothetical reasoning, signifying the transition from concrete logic to the capacity for abstract thought.
Step-by-step explanation:
The core change that happens in the formal operational stage is a. Abstract thinking and hypothetical reasoning. During this stage, which typically begins around the age of 11, children start to think logically not only about concrete events but also about abstract ideas and hypothetical situations. This ability allows for advanced problem-solving, considering multiple solutions, and testing theories. Unlike the earlier concrete operational stage, where thought processes are tied to concrete and tangible objects and experiences, the formal operational stage enables individuals to reason about abstract concepts such as love, justice, and freedom. They become capable of hypothetical thinking, which is essential for planning for the future, scientific reasoning, and moral reasoning.