Explanation: The deviation ratio is a fundamental parameter in frequency modulation (FM) that characterizes the design of an FM system. It is defined as the ratio of the maximum frequency deviation of the carrier (how much the carrier frequency shifts) to the highest audio modulating frequency expected in the system. * **Option B is correct** because it accurately defines the deviation ratio as the maximum carrier frequency deviation divided by the highest audio modulating frequency. This ratio helps determine the bandwidth requirements and the quality of the FM signal. * **Option A is incorrect** because it describes a simple ratio of audio to carrier frequency, which is not the definition of deviation ratio. * **Option C is incorrect** as it's the inverse of Option A and also not the correct definition. * **Option D is incorrect** as it compares two audio frequencies, which has no direct relation to the carrier frequency deviation or the deviation ratio.