Explanation: In a Pulse-Width Modulation (PWM) system, the modulating signal varies the **pulse duration** (or width). Here's why: PWM works by taking a series of constant-amplitude, constant-frequency pulses and changing the *on-time* (duration) of each pulse in proportion to the instantaneous amplitude of the modulating signal. A higher instantaneous modulating signal voltage results in a wider, longer-duration pulse, while a lower voltage results in a narrower, shorter-duration pulse. The amplitude and repetition rate (frequency) of the pulses remain constant. * **A) Pulse frequency:** This describes Pulse Frequency Modulation (PFM), where the *repetition rate* of the pulses is varied, not their width. * **C) Pulse amplitude:** This describes Pulse Amplitude Modulation (PAM), where the *amplitude* (height) of the pulses is varied. * **D) Pulse intensity:** While related to power and thus implicitly affected by pulse duration and amplitude, "pulse intensity" is not the direct parameter being varied by the modulating signal in standard modulation schemes.