Explanation: The term for the maximum frequency deviation of an FM signal divided by the maximum audio modulating frequency is the **deviation ratio**. This ratio is a specific design parameter for an FM system. It defines the ratio of the *maximum permissible frequency deviation* (how much the carrier can shift) to the *maximum modulating frequency allowed* (the highest audio frequency component the system is designed to handle). For example, in narrowband FM used by amateur radio, a typical maximum deviation might be 5 kHz, and the maximum audio frequency around 3 kHz, resulting in a deviation ratio of approximately 1.67. While **modulation index** is a general term for the ratio of *any given* frequency deviation to an *instantaneous* modulating frequency ($\Delta f / f_m$), the question's specific use of "maximum deviation" and "maximum audio modulating frequency" points to the fixed system design limits, which is precisely what the deviation ratio describes. "Deviation index" and "modulation ratio" are not standard terms in this context.