The correct response to explain 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 option emphasizes that the extensive training and certification needed to enter these medical professions create barriers to entry, resulting in a lower supply of qualified labor. As a result, when demand for their services is high, the limited supply can contribute to relatively high wages.