The explanation that best accounts for 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 means that the extensive education, training, and certification processes required to enter these medical fields limit the number of individuals who can and are willing to qualify for these roles, which contributes to higher wage levels due to the scarcity of qualified professionals in the labor market.