Explanation: A deviation meter is specifically designed to measure the peak frequency deviation of an FM (Frequency Modulation) signal. In FM, the instantaneous frequency of the carrier varies in proportion to the amplitude of the modulating signal. The deviation meter quantifies the maximum shift, or "swing," of the carrier frequency above and below its assigned center frequency. This measurement is crucial for ensuring the transmitter operates within its authorized bandwidth and avoids over-deviation, which can cause spurious emissions and interference. * **A) Measure the number of sidebands:** While deviation influences the spectral width and the number of significant sidebands, a deviation meter does not directly count them. * **B) Measure the modulation index:** The modulation index is a *calculated* value (peak deviation divided by the highest modulating frequency), not directly measured by a deviation meter. The meter provides one component of that calculation. * **D) Measure the carrier frequency:** The carrier frequency is the nominal center frequency, typically measured by a frequency counter or spectrum analyzer, not the deviation meter, which measures the *change* from that center.