Explanation: The velocity factor (VF) of a transmission line represents the ratio of the speed at which an electromagnetic wave propagates through the line compared to the speed of light in a vacuum. This speed is fundamentally determined by the properties of the insulating material separating the conductors, known as the dielectric. Electromagnetic fields travel predominantly through this dielectric. The dielectric constant (or relative permittivity) of the material directly influences how quickly the wave can propagate; a higher dielectric constant slows the wave down, resulting in a lower velocity factor. Therefore, different dielectric materials (e.g., polyethylene, PTFE, air) will yield different velocity factors for the transmission line. Termination impedance (A) affects reflections and standing waves, not the intrinsic propagation speed. Line length (B) affects total attenuation and phase shift, but not the speed per unit length. Center conductor resistivity (D) primarily contributes to ohmic losses (attenuation), not the speed of the electromagnetic wave itself.