Aseries of legislative acts passed under the grant administration in 1870 and 1871 authorized federal protection for black suffrage through the use of the army against the ku klux klan. together, these were known as the acts. a. klan b. grant c. reconciliation d. force

The right answer is D. Force (Force Acts)

You can also find another name for them: Enforcement Acts.
The aswer A is probably introduced for confusion, as the acts are also known colloquially as the Ku Klux Klan Acts, as they were in fact devised against the Ku Klux Klan.
In particular, one of the acts, the third in chronological order, which gave the president powers to counteract the KKK with force, became known as the Ku Klux Klan Act (in singular, Ku Klux Klan Acts refer all all three acts). We could take answer A into consideration if it said "Ku Klux Klan" and not only "Klan"- but in any case, the answer D is the more proper term for those acts.