The correct answer is B. Madison is answering Anti-Federalist concerns over a centralized national government.
In this quote, Madison emphasizes that the proposed Constitution is a mix of both national and federal elements, which addresses the Anti-Federalist concerns about the potential for a too-powerful centralized national government. By framing it as a compromise, he seeks to reassure those who feared that a strong national government would threaten the rights and sovereignty of individual states.