“...But the great security against a gradual concentration of the several powers in the same department, consists of giving those who administer each department, the necessary constitutional means, and personal motives, to resist encroachment of the others.” Source: James Madison, President of the United States (1809-1817), The Federalist Papers: No. 51, published February 8, 1788

In this excerpt, Madison is referring to which ONE of the following US government foundational principles?
Select one:

a.
Social contract

b.
Federalism

c.
Natural rights

d.
Separation of powers