Final answer:
Cardio fitness involves the efficiency of the respiratory, circulatory, and muscular systems, but not the digestive system, which is not directly involved in this process.
Step-by-step explanation:
​Cardio fitness reflects the efficiency of a person's ability to intake, transport, and utilize oxygen during exercise or physical activity. It involves several systems, including the respiratory system which is responsible for the intake of oxygen, the circulatory system which transports oxygen to the muscles and organs, and the muscular system which uses oxygen for movement and endurance. However, the digestive system, while important for providing the nutrients that the body needs to produce energy, is not directly involved in the process of cardio fitness. Therefore, the correct answer is d) Digestive system.