Final answer:
In Federalist No. 51, James Madison addressed Anti-Federalist concerns by promoting the system of checks and balances, arguing that it, along with the separation of powers, would prevent any one branch of government from becoming too powerful. The correct option is B.
Step-by-step explanation:
In Federalist No. 51, the argument used to address the concerns of the Anti-Federalist is b) Checks and balances. James Madison, under the pseudonym Publius, explained that each branch of the government would have a check over the power of the other branches. Furthermore, the presence of strong separation of powers between branches ensured that no single branch could become too powerful.
By instituting this system, Madison assured that the ambition of each branch to hold onto its powers would serve as a self-regulating force, preventing overreach or tyranny.
Moreover, the federal system, mentioned by Madison, provided an additional layer of security against governmental overreach by distributing powers between state and national governments. Thus, the Federalist No. 51 essay aimed to reassure those concerned about an overly powerful national government that the proposed Constitution had built-in safeguards through checks and balances and the separation of powers.