Explanation: Sparking at key contacts occurs due to the rapid interruption of current through an inductive load in the keying circuit. When the contacts open, the inductor resists this sudden change, generating a high voltage "kickback" spike (L di/dt) that can arc across the opening gap. A key-click filter, typically an RC (resistor-capacitor) network, is connected *across* (in parallel with) the key contacts. This filter provides a path for the inductive energy to dissipate when the key opens. The capacitor charges, and the resistor limits the current, effectively shunting the high voltage spike and preventing the arc. This not only reduces contact wear but also helps shape the keying waveform, minimizing "key clicks" – unwanted broadband noise generated by the sharp make and break of the key. Connecting a filter in series (B) would impede normal keying. A high-pass filter (C) is not designed for shunting inductive spikes in this manner. Increasing the contact space (D) does not address the fundamental issue of the high voltage spike.