Article 1 Section 8: makes D very much a Federal Domestic Concern
Workers subjected to unjust treatment is also a Federal Domestic Concern. I don't know where it is in the constitution, but I do know it is somewhere.
The bill of rights is actually the first 10 amendments. No other ten laws have garnered as much attention by the supreme court as those first 10 amendments. It is one of the deepest concerns of congress in protecting the citizens it governs.
The answer you want is C. There is no domestic concern in the disputed islands in the South China Sea.