The most efficient vertebrate respiratory system is that of b) birds.
Birds have a specialized respiratory system that allows for continuous airflow through their lungs, which is more efficient than the systems found in mammals, reptiles, amphibians, or humans. This system includes air sacs that facilitate a unidirectional flow of air, ensuring that fresh air is always available for gas exchange. This design enables birds to meet the high metabolic demands of flight, providing them with the necessary oxygen even at high altitudes where the air is thinner. In contrast, other vertebrates typically have bidirectional flow lungs, which means that the air mixes with stale air, making their systems less efficient when it comes to oxygen uptake.