Explanation: The velocity factor (VF) of a transmission line describes how fast an electrical signal travels through the line compared to the speed of light in a vacuum. Specifically, it is the ratio of the propagation velocity of the electromagnetic wave *on the transmission line* to the velocity of light in a vacuum. Since the dielectric material between the conductors of a transmission line (like the insulation in coaxial cable) slows down the electromagnetic wave, the velocity factor is always less than 1. This factor is crucial for determining the physical length needed for a specific electrical length, such as when building antennas or impedance matching sections. * **A) The ratio of the characteristic impedance of the line to the terminating impedance.** This describes an impedance mismatch, which is related to Standing Wave Ratio (SWR), not velocity factor. * **C) The velocity of the wave on the transmission line multiplied by the velocity of light in a vacuum.** This mathematical operation would result in a value with units of (speed)^2, which is incorrect for a dimensionless factor. * **D) The index of shielding for coaxial cable.** Shielding effectiveness is a separate characteristic indicating how well a cable resists external interference or signal leakage.