Explanation: A frequency counter's primary function is to measure the frequency of an oscillating electrical signal. It achieves this by digitally counting the number of cycles of the input signal that occur within a very precise, predefined time interval (often called the "gate time"). The frequency is then displayed as cycles per second (Hertz). Alternatively, for very low frequencies, some counters can measure the period of the waveform (the time it takes for one complete cycle) and then calculate the frequency as the reciprocal of that period (frequency = 1/period). Therefore, option D accurately describes its operation: measuring the time between events (like cycles) to determine frequency, which is inherently the reciprocal of time. Option A describes a frequency calibrator or marker generator, which *generates* reference frequencies, rather than measuring them. Option B describes a heterodyne frequency meter, an older method of comparing an unknown frequency to a known one until "zero beat" is achieved. Option C describes an FM deviation meter or modulation monitor, a specialized instrument for analyzing the frequency shift in an FM signal, not the general function of a frequency counter.