Mental injury is difficult to verify because the law requires what?

Mental injury is often difficult to verify because it cannot be shown through physical evidence. Since the question mentions that the law requires something, it becomes apparent that the law has to establish the link between parental behavior and harm to the child. Witness testimonies and psychological assessments may support this link, but they are not the main requirement for the law to consider a mental injury as valid. Therefore, option A is the best answer. Option B is incorrect because mental injury does not always have physical evidence. Option C may be helpful in providing additional evidence, but it is not the main requirement. Option D may be useful, but it is not the sole requirement for verifying mental injury.