What component in a magneto reduces arcing at the points?

The component in a magneto that reduces arcing at the points is a capacitor. In this context, the capacitor serves to store electrical energy and then discharge it, helping to smooth out the electrical current flowing through the ignition system. When the ignition points open, the voltage can spike, leading to arcing or sparking at the contacts. The capacitor mitigates this effect by absorbing some of the energy and providing a more stable discharge, which results in less wear on the points and a more reliable ignition spark.

The other components listed, while they play important roles in various electrical systems, do not specifically address the problem of arcing at the points in the same way that a capacitor does. For instance, a diode primarily allows current to flow in one direction and is not involved in smoothing or reducing voltage spikes. A resistor limits the flow of current but does not directly help in reducing arcing. The coil, while essential for generating high voltage in the ignition system, also does not impact the arcing at the contacts directly. Thus, the capacitor is the key component for addressing this issue in a magneto system.