The response that best explains the relatively high wages earned by certain medical professionals, like nurse practitioners and physicians, is:
The training and certification required for these professions lead to a relatively low quantity of labor supplied at any given wage.
This is because the extensive education, training, and certifications necessary to become a nurse practitioner or physician create barriers to entry in these fields. As a result, the supply of qualified professionals is limited, which can lead to higher wages due to the demand for their specialized skills and services.