Marcy's employer, Sam, asks her to commit perjury when she testifies in a lawsuit against him for selling defective products in his appliance store. Marcy refuses, and Sam promptly fires her. In most jurisdictions, which of the following would likely provide Marcy with a cause of action against Sam for the firing?

a. Breach of employment-at-will contract

b. Violation of the due process clause of the U.S. Constitution

c. Wrongful discharge in violation of public policy

d. Violation of First Amendment right to free speech contained in the U.S. Constitution