Energy transfer is inefficient between trophic levels primarily because molecules are not fully digested from each trophic level. Some energy is lost as heat, and not all biomass is consumed or converted into energy for the next trophic level. Therefore, the best choice is:
molecules are fully digested from each trophic level.
However, note that the answer options provided seem to suggest that the misunderstanding of "fully digested" is necessary for inefficiency, when in fact, the inefficiency is due to energy loss through metabolic processes and other factors like respiration and waste.