During the Freedom Summer campaign of 1964 in Mississippi, three civil rights workers were kidnapped.
three KKK members were found dead.
three civil rights workers were found dead.
three civil rights workers attacked the KKK.

I'm pretty sure it is the first one

Explanation:

A

C. Three civil rights workers were found dead

Explanation:

basic principles: federalism, separation of powers, checks and balances, judicial review, representative democracy, limited government, rule of law, individual rights and responsibilities, consent of the governed, majority rule, popular sovereignty, equal protection.