Correct answer: That the violence of factions must be controlled.
The opening line by James Madison in Federalist #10 states: "Among the numerous advantages promised by a well constructed Union, none deserves to be more accurately developed than its tendency to break and control the violence of faction." He adds (still in his introduction), that "the friend of popular governments ... will not fail, therefore, to set a due value on any plan which ... provides a proper cure" for the problem of factions.
The Federalist Papers were essays written by James Madison, Alexander Hamilton, and John Jay, which originally appeared in serial fashion in several newspapers. They used the pseudonym "Publius" in sending them for publication. These 77 essays, plus 8 more, were then published in a 2-volume set in 1788, under the title,<em> The Federalist: A Collection of Essays, Written in Favour of the New Constitution, as Agreed upon by the Federal Convention, September 17, 1787.</em>