Final answer:
The Supreme Court's ruling in Citizens United v. Federal Election Commission permits corporations to spend unlimited funds on political advertising, as it's considered a form of protected free speech under the First Amendment. The correct answer is option a).
Step-by-step explanation:
A corporation spending millions of dollars on advertising for a candidate's presidential campaign is political speech according to the Supreme Court's ruling in Citizens United v. Federal Election Commission. The Court's decision in 2010 determined that the government cannot ban political spending by corporations in candidate elections because it violates the First Amendment's free speech clause.
The Citizens United ruling allows corporations and unions to spend unlimited amounts of money on elections, as long as the spending is independent of the candidates themselves. While this decision has led to a significant increase in election spending, it has also been the subject of controversy and debate regarding the influence of wealth on the political process.