Final answer:
A. Accessibility
Extra billing or user fees compromise the principle of Accessibility in healthcare systems, as they create a financial barrier that can make healthcare less accessible to those unable to afford additional costs.
Step-by-step explanation:
The principle that applies to extra billing or user fees within a healthcare context is Accessibility. These terms are related to the additional charges that patients may have to pay for healthcare services, on top of what is covered by their insurance or national health coverage. Within certain health care systems, notably Canada's, the principle of Accessibility is crucial, as it mandates that healthcare must be accessible to all residents without financial or other barriers. Thus, extra billing or user fees would compromise this principle by making healthcare less accessible to those who may not be able to afford these additional costs.