Baby teeth, particularly baby incisors, tend to be square and flat.
While the term "flat" is relative when describing teeth, baby incisors (the front teeth) are generally flatter and more square compared to the more rounded edges of adult incisors. Adult molars also possess flat surfaces, but the question seems to be referring to the overall shape and structure rather than a specific biting surface. Therefore, baby incisors best fit the description of a "flat" tooth.
