Answer: C. A case involving the constitutionality of a restaurant chain refusing to serve specific customers.
Explanation: Federal courts have jurisdiction over cases involving federal laws, constitutional issues, and disputes between parties from different states (diversity jurisdiction). In this case, the issue of the constitutionality of a restaurant chain refusing to serve specific customers involves constitutional rights, which falls under the jurisdiction of federal courts.
Federal courts have the authority to interpret and apply the U.S. Constitution, so if the case involves a violation of constitutional rights, such as discrimination or a denial of due process, it would be within the jurisdiction of federal courts.
In this scenario, if the restaurant chain's refusal to serve specific customers is challenged on constitutional grounds, such as violating the Equal Protection Clause or the First Amendment rights of the customers, a federal court would have the authority to hear the case and make a ruling.