Explanation: To operate a Marconi antenna (a quarter-wave vertical) on a lower resonant frequency, its electrical length must be increased. A lower frequency corresponds to a longer wavelength, so the antenna needs to appear longer to resonate efficiently. Adding an inductor in series with the antenna introduces inductive reactance. If the antenna is physically shorter than a quarter-wavelength for the desired lower frequency, it will exhibit capacitive reactance. The added inductive reactance cancels some of this capacitive reactance, effectively extending the antenna's electrical length without physically adding more wire. This allows the antenna to achieve resonance at a lower frequency. Conversely, adding a capacitor in series (A) would introduce capacitive reactance, making the antenna appear electrically *shorter* and raising its resonant frequency. Options C and D are incorrect because a screen grid bypass capacitor (C) is an internal amplifier component, and an RF choke across the feed line (D) would not adjust antenna resonance but rather block RF or detune the line.