The landmark Supreme Court case Brown v. Board of Education (1954) fundamentally changed American society by ruling that:

The correct answer to the question "The landmark Supreme Court case Brown v. Board of Education (1954) fundamentally changed American society by ruling that:" is Option B: Segregation in public schools was unconstitutional.

The other options — A) Separate but equal facilities were constitutional, C) Affirmative action policies were mandatory, D) Voting rights tests were permissible — are incorrect for this GED Exam Prep question. As explained above: Brown v. Board of Education overturned the "separate but equal" doctrine established in Plessy v. Ferguson, declaring that racial segregation in public education was inherently unconstitutional.