Final answer:
The nurse's primary and legal responsibility when suspecting child abuse is to document observations accurately, which is essential for reporting the case to the appropriate authorities.
Step-by-step explanation:
When a nurse suspects physical abuse in a child, their primary and legal responsibilities include conducting a thorough physical examination, accurately documenting observations, and importantly, reporting the suspicions to the appropriate authorities. Of the choices presented, the nurse's top legal responsibility is to document observations accurately (option b). Contacting a child's teacher (option c) or notifying the child's peers (option d) is inappropriate and can breach confidentiality. Furthermore, while conducting a physical examination (option a) is part of the assessment process, the legal obligation of a nurse is to ensure that suspected abuse is properly recorded and reported to the responsible agencies.