The correct answer is C: Resistance and reactance. The two families of circles and arcs that make up a Smith chart are resistance and reactance. Resistance is represented by circles, and reactance is represented by arcs. The Smith chart consists of two families of curves: circles of constant resistance and arcs of constant reactance. Resistance circles are horizontal circles centered on the horizontal axis. Reactance arcs are curved lines - inductive reactance in the upper half, capacitive reactance in the lower half. Together, these two families create a grid where any impedance (R + jX) can be plotted. The intersection of a resistance circle and a reactance arc gives you a specific impedance point. This is the fundamental structure of the Smith chart.