A _ child, she was soon bored in class; she already knew more mathematics than her junior school teachers.

Explanation

The correct word to fill in the blank is "precocious." The sentence would be: "A precocious child, she was soon bored in class; she already knew more mathematics than her junior school teachers." "Precocious" means having developed certain abilities or talents at an early age, often beyond what is typical for children of that age. In the given context, it describes a child who is intellectually advanced and shows great maturity and understanding at a young age. Precocious refers to a child having developed certain abilities or inclinations at an earlier age than is usual or expected.