Explanation: Cross-modulation interference occurs when a strong, unwanted radio signal enters a receiver along with a desired signal. Due to non-linearities in the receiver's RF amplifier stage, the modulation (e.g., audio or data) of the strong, unwanted signal is inadvertently transferred onto the carrier of the desired signal. The operator then hears the desired station, but with the superimposed modulation from the interfering signal, making it difficult to understand. A) This describes general interference, not the specific mechanism of cross-modulation where one signal's *modulation* is transferred to another. B) Audio rectification is a different phenomenon where strong RF signals are directly converted into audible frequencies by non-linear components, not the transfer of modulation between two distinct RF carriers. D) Harmonic distortion refers to the generation of spurious signals at integer multiples of a transmitted frequency, which is a characteristic of a single transmitter's output, not an interaction between two signals in a receiver.