Which of the following actions should you take if a neighbor tells you that your station’s transmissions are interfering with their radio or TV reception?

If a neighbor reports interference, the very first step you should take is to verify the operation of your own station. By checking if your transmissions interfere with your own television or radio tuned to the same channel, you can determine if the problem is caused by spurious emissions from your transmitter or by a vulnerability in your neighbor's equipment. If your equipment operates cleanly at home, the interference is most likely due to fundamental overload or inadequate shielding in the neighbor's consumer electronics. This gives you a baseline to help them troubleshoot the issue, usually by adding filters to their devices. The other options are incorrect or nonsensical. You do not need to contact the FCC immediately, as most interference issues can be resolved cooperatively. Furthermore, a 'harmonic doubler' is a fictitious device in this context; generating harmonics is exactly what you want to avoid to prevent interference.