Final answer:
The correct answer is option a. allows corporations and labor unions to spend unlimited funds on campaigns.
Step-by-step explanation:
The Supreme Court decision in Citizens United v. Federal Election Commission (2010) allows corporations and labor unions to spend unlimited funds on campaigns. This decision overturned previous campaign finance reform legislation and ruled that contributions from corporations and unions to Political Action Committees (PACs) are a form of protected free speech under the First Amendment.
As a result, corporations and labor unions can now spend unlimited sums of money to support or oppose political candidates as long as the spending is independent of the candidates.