Answer:
C. Systems in other countries, where students can leave school with minimal debt, can serve as models for solutions that we can adapt in the U.S.
Step-by-step explanation:
The sentence can exist without the part enclosed in commas (where students can leave school with minimal debt). You would be looking for an explaination of whatever the first part is. For example, if you didn't know about the systems in other countries, you then add the explaining part.