Under Jean Piaget's theory of cognitive development, during the transition from the 'Concrete Operational Stage' to the 'Formal Operational Stage', when deeply analyzing the qualitative changes in children's thinking, which of the following major cognitive abilities fully develops only in the fourth stage?

According to Jean Piaget's theory of cognitive development, the most important characteristic of the 'Formal Operational Stage' (from roughly 11 years to adulthood) is the development of 'hypothetico-deductive reasoning' and abstract thinking. At this level, a child does not rely solely on direct or concrete objects, but can draw logical conclusions without seeing them and is capable of solving scientific problems.