What parameter describes the interaction of a load and transmission line?

The reflection coefficient is the parameter that describes the interaction between a load and a transmission line. It quantifies how much of an electromagnetic wave is reflected by an impedance discontinuity when a signal reaches the end of the line. When the load impedance perfectly matches the characteristic impedance of the transmission line, the reflection coefficient is zero, meaning all power is absorbed by the load and none is reflected back to the source. Other options like characteristic impedance, velocity factor, and dielectric constant describe the physical and electrical properties of the transmission line itself, not the dynamic interaction that occurs when the line meets a connected load.