The statement 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 leads to a relatively low quantity of labor supplied at any given wage.
This is because the rigorous education, training, and licensure requirements create a barrier to entry, which reduces the quantity of individuals who can work in these professions. As a result, with a low supply of qualified medical professionals and a consistent demand for their services, wages tend to be higher.