Why In FM, the deviation ratio is the maximum permitted carrier frequency deviation divided by the highest audio modulating frequency the system will pass. For a typical narrowband amateur FM repeater channel, 5 kHz deviation over a 3 kHz maximum audio frequency gives a deviation ratio of about 1.67. It is essentially the modulation index evaluated at the worst case: peak deviation and the top of the audio passband, so it describes the whole system rather than one instant.
Watch out The choices involving the carrier center frequency are a trap because the carrier frequency has nothing to do with it. What matters is how far the carrier swings, not where it sits, which is why an FM signal's bandwidth is the same on 29 MHz as on 440 MHz.
Deviation ratio = max deviation over max audio frequency. Both numbers are maximums, and the carrier frequency never appears.