Explanation: A low-pass filter network is designed to pass frequencies below a certain cutoff frequency and attenuate frequencies above it. This fundamental characteristic makes them widely applicable in various electronic circuits: * **Ripple filter in power supplies:** After rectification, AC ripple remains on the DC output. A low-pass filter smooths this out by allowing the DC (0 Hz) to pass while attenuating the higher-frequency ripple components. * **Harmonic suppression filters in antenna systems:** Transmitters generate harmonics at multiples of the desired operating frequency. A low-pass filter between the transmitter and antenna allows the desired fundamental frequency to pass while blocking these higher-frequency harmonics, preventing interference to other services. * **Line noise filters:** These filters prevent high-frequency electrical noise from entering or leaving equipment via power lines. * **Tone control systems:** In audio circuits, a low-pass filter can reduce treble, emphasizing lower frequencies. * **Crossover networks:** In speaker systems, low-pass filters direct only low frequencies to the woofer. * **Output filters of detector circuits:** After detection in a receiver, a low-pass filter separates the desired audio (low frequency) from any remaining RF or IF components (high frequency). Since a low-pass filter serves these diverse functions, all the options listed represent valid applications.