If f_{m} is the modulation frequency in FM, the modulation index is proportional to

- f
_{m} - 1/f
_{m}^{2} - f
_{m}^{2} - 1/f
_{m}

**Answer**

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/f_{m}

where f_{m} is the modulation frequency and Δf is the frequency deviation.

**The correct option is D.**