Final answer:
The formal operational stage in Piaget's theory is where trial and error are replaced by problem-solving abilities, characterized by abstract thinking and dealing with hypothetical situations.
Step-by-step explanation:
Trial and error are replaced by the ability to problem solve in the formal operational stage of Piaget's developmental theory. This is the final stage in Piaget's theory of cognitive development, starting around age 11, during which children develop the ability to think abstractly and reason about hypothetical situations. In this stage, adolescents can use abstract thinking to problem solve and consider multiple outcomes, which is a significant shift from the concrete operational stage where their thinking was limited to real events and logical reasoning was only beginning to be developed.