Explanation: In Pulse-Position Modulation (PPM), the amplitude and duration (width) of the individual pulses are kept constant. Instead, the instantaneous amplitude of the modulating signal determines the *position* or timing of each pulse within a given time window. A larger modulating signal amplitude might shift a pulse earlier, while a smaller amplitude shifts it later, relative to a reference point. Therefore, the modulating signal directly varies the time at which each pulse occurs. * **A) The number of pulses per second:** This is not how PPM works; the pulse repetition rate is typically constant or controlled separately. * **C) Both the frequency and amplitude of the pulses:** In PPM, the pulse amplitude is constant. While the *rate* of pulses might imply a frequency, the pulses themselves don't have varying frequency. This describes aspects of analog modulation (AM/FM) rather than pulse modulation. * **D) The duration of the pulses:** Varying the duration (or width) of pulses describes Pulse-Width Modulation (PWM), another distinct type of pulse modulation.