Final answer:
In the given scenario where a 16-year-old child requires emergency medical treatment for a broken tooth, the child's parents are the ones who can provide an expressed contract for treatment. Neither the teacher, neighbor, nor the child's dentist have the legal authority to consent to the medical treatment of a minor in this context.
Step-by-step explanation:
In the context of a 16-year-old child who has broken a tooth and requires emergency treatment, an expressed contract for such medical care can typically be provided by the child's legal guardians. Among the available options presented, the child's parents are the most appropriate to provide consent for medical treatment. Teachers, neighbors, and even the child's dentist, while possibly involved in the child's life, are not legally authorized to consent to medical treatment on behalf of a minor without explicit authority or extenuating circumstances such as implied consent in an emergency where parents are unavailable.
Moreover, parents are generally responsible for their minor children's medical decisions, including consenting to treatment. It is only the parents' consent that is considered legally valid in the case presented unless there are particular state laws or court orders that say otherwise.