According to Jean Piaget's theory of cognitive development, when a child enters the 'Formal Operational Stage', which of the following is the most primary and specific cognitive capacity developed in their thinking process?

In Jean Piaget's theory of cognitive development, the most prominent feature of the 'Formal Operational Stage' (from approximately 11 years to adulthood) is the development of abstract thinking, hypothetical-deductive reasoning, and scientific thinking. In this stage, children become capable of solving problems based solely on verbal or logical propositions without the presence of any concrete materials. For more information on this topic, you can refer to the Wikipedia reference.