Explanation: The modulation index is a key parameter in frequency modulation (FM). It quantifies the amount of frequency deviation relative to the modulating frequency. Specifically, it is defined as the ratio of the frequency deviation (how much the carrier frequency shifts from its center frequency) to the modulating frequency (the frequency of the audio or data signal causing the deviation). This ratio is crucial for determining the bandwidth of an FM signal and is represented by the formula $\beta = \Delta f / f_m$, where $\Delta f$ is the frequency deviation and $f_m$ is the modulating frequency. Option A correctly describes this fundamental ratio. Option B refers to a non-existent term. Option C describes the signal quality, not the modulation index. Option D describes the ratio of *maximum* deviation to the *highest* modulating frequency, which is used to calculate the *maximum* modulation index or bandwidth, but not the general definition of the modulation index itself at any given instant.