Final answer:
Children begin to think hypothetically and develop complex cognitive abilities in the formal operational stage, typically starting around 11-14 years of age.
Step-by-step explanation:
Children begin to think hypothetically and have a greater ability for complex thought during the formal operational stage of development, which is option c. 11-14 years. According to Piaget's theory, this stage is characterized by the ability to understand abstract concepts and engage in hypothetical thinking.
At this age, adolescents are able to use abstract thinking to problem-solve, consider alternative solutions, and test these solutions.