Explanation: A harmonic is a frequency that is an integer (whole number) multiple of an original or fundamental frequency. For instance, if a signal's fundamental frequency is 10 MHz, its second harmonic is 20 MHz (2 x 10 MHz), and its third harmonic is 30 MHz (3 x 10 MHz). Harmonics are a critical concept in amateur radio because transmitters can generate these unwanted signals, potentially causing interference on other frequency bands. Option B describes power dissipation or resistive heating, not a frequency relationship. Option C refers to the internal electrical resistance of a power source. Option D relates to the efficiency of AC power transfer in electrical circuits. None of these definitions describe a multiple of a frequency.