What factors determine the characteristic impedance of an open-wire transmission line?

The characteristic impedance of an open-wire (parallel-conductor) transmission line is determined solely by its physical dimensions and the insulating material (dielectric) between the wires. Specifically, it depends on the distance between the centers of the two conductors and the diameter (or radius) of those conductors. Increasing the spacing between the wires increases the characteristic impedance, while increasing the diameter of the wires decreases it. The formula relies entirely on the ratio of this spacing to the conductor diameter. Options suggesting that frequency or line length determine characteristic impedance are incorrect. Characteristic impedance is a fixed property of the line's manufacturing and geometry, independent of how long the line is or what signal frequency is passing through it.