The ratio of the audio modulating frequency to the center carrier frequency
The ratio of the maximum carrier frequency deviation to the highest audio modulating frequency✓Correct answer
The ratio of the carrier center frequency to the audio modulating frequency
The ratio of the highest audio modulating frequency to the average audio modulating frequency
Explanation
Deviation ratio is the maximum carrier deviation the system allows divided by the highest audio frequency it passes. It differs from modulation index in being a fixed design figure evaluated at those two limits, whereas the modulation index varies with whatever audio is present at the moment.