Explanation: Standing waves on a transmission line are a direct result of reflections. When the impedance of the load (ZL) does not perfectly match the characteristic impedance (Z0) of the transmission line, not all of the radio frequency (RF) energy transmitted down the line is absorbed by the load. Instead, a portion of the energy is reflected back towards the source. This reflected energy interferes with the forward-traveling energy, creating stationary patterns of voltage and current maximums and minimums along the line, which are called standing waves. Therefore, the presence of standing waves clearly indicates that some of the energy is not absorbed by the load (D). If all energy were being delivered to the load (A), there would be no reflections and thus no standing waves. Options B and C describe conditions for a perfectly matched line (ZL = Z0) where all energy is absorbed, resulting in no standing waves. The term "surge impedance" in B is likely referring to characteristic impedance; if source and load impedances equal the characteristic impedance, there would be no standing waves.