Formal instruction in the scientific method usually begins when pupils are between 11 and 13 years of age. at this point, students are capable of the abstract thinking and deductive reasoning characteristic of piaget's stage of cognitive development.

The correct answer is the formal operational stage of cognitive development. 

According to Piaget's theory of cognitive development, the formal operational stage of development is characterized by the ability to think abstractly, engage in deductive reasoning and logically investigate scientific hypotheses. This stage of development typically occurs at age 11 and involves an advanced type of thought process.