Which case involved the freedom of speech and the regulation of campaign finance?

The case that involved the freedom of speech and the regulation of campaign finance is indeed Citizens United v. Federal Election Commission. This landmark decision by the Supreme Court, decided in 2010, held that the government cannot limit independent expenditures for political communications by corporations, associations, or individuals. The Court reasoned that such limits would violate the First Amendment's guarantee of free speech. The ruling significantly impacted campaign finance laws, essentially allowing for unlimited spending by outside groups in elections, which the Court classified as a form of protected speech.

In contrast, the other cases mentioned do not primarily focus on freedom of speech or campaign finance. Fisher v. University of Texas deals with affirmative action in college admissions. Roe v. Wade is centered on women's reproductive rights and privacy issues, while Plessy v. Ferguson established the "separate but equal" doctrine regarding racial segregation. Each of these cases addresses crucial civil rights and legal principles but does not pertain to the intersection of speech and campaign finance like Citizens United does.