Final answer:
The U.S. Supreme Court case Brown v. Board of Education in 1954 overturned the 'separate but equal' doctrine of Plessy v. Ferguson, declaring racial segregation in public schools unconstitutional.
Step-by-step explanation:
In 1954, the United States Supreme Court case that overturned the Plessy v. Ferguson decision declaring the doctrine of "separate but equal" constitutional was a) Brown v. Board of Education. The landmark Brown v. Board case led to the unanimous decision that declared racial segregation in public schools as inherently unequal and thus unconstitutional, contradicting the precedent set by Plessy v. Ferguson. This case was a significant milestone in the civil rights movement, contributing to the eventual desegregation of schools across the nation.