The correct answer is D: It does not depend on the RF carrier frequency. The modulation index of a phase-modulated emission does not depend on the RF carrier frequency. The modulation index is determined by the phase deviation and modulating frequency, not the carrier frequency. In phase modulation (PM), the modulation index is the peak phase deviation in radians. This is determined by how much the phase changes in response to the modulating signal, which is independent of the carrier frequency. Whether you're phase modulating at 1 MHz or 100 MHz, if the phase deviation and modulating frequency are the same, the modulation index is the same. This is different from some other modulation parameters that do vary with carrier frequency.