If fm is the modulation frequency in FM, the modulation index is proportional to

1. fm
2. 1/fm2
3. fm2
4. 1/fm

The value of the modulation index indicates by how much the modulated variable varies around its unmodulated level. It relates to variations in the carrier frequency:

h = Δf/fm

where fm is the modulation frequency and Δf is the frequency deviation.

The correct option is D.