The best answer, of those given as options here, is D.
The case of Prigg v. Pennsylvania involved a man who was convicted of returning an allegedly fugitive slave to slavery without following proper procedures for ascertaining that she was a slave. Pennsylvania had enacted a law making it illegal to do this. Thus, the case clearly was about how fugitive slaves should be treated.
The case was also about the enforcement of federal law as opposed to state law. The lawyers for Prigg argued that the statute that Pennsylvania had passed violated federal law and the Constittuion. They argued that it went against Article IV, Section 2 of the Constitution, which has to do with fugitive slaves. They also argued that the Pennsylvania law was in conflict with the Fugitive Slave Act of 1793. This means that the case was at least partly about whether federal law or state law would be enforced.
If both A and B are correct, then D must be the correct answer.