Which required Supreme Court case most directly strengthened the national side of the federal-national balance described by Madison?

McCulloch v. Maryland upheld implied congressional power to create a national bank and held that a state could not tax it, strengthening national authority and federal supremacy, so D is correct. Choice A is tempting because Lopez is a major federalism case, but it limited Congress's Commerce Clause power, favoring the states. Choice B, Baker v. Carr, let federal courts hear apportionment challenges but did not establish implied national powers. Choice C, Shaw v. Reno, limits state redistricting on equal-protection grounds, not the scope of national legislative power.