Explanation: In Frequency Modulation (FM), the modulation index is a crucial parameter that describes the extent of frequency variation of the carrier signal relative to the modulating signal's frequency. It is defined as the ratio of the maximum frequency deviation of the carrier (how far its frequency shifts from its center) to the maximum frequency of the modulating signal (e.g., the highest audio frequency). Therefore, option A, "The maximum carrier deviation divided by the maximum modulation frequency," accurately defines the modulation index. A higher modulation index indicates a wider bandwidth for the FM signal. Option B is incorrect because it relates modulation frequency to carrier frequency, which is not the definition of modulation index. Option C incorrectly suggests multiplication and involves the carrier frequency, not deviation. Option D introduces "pre-emphasis," which affects the frequency response of the audio before modulation but is not part of the modulation index calculation itself.