What reasoning correctly pairs a body system with its relationship to the digestive system?

Respiratory system.

Explanation:

This is the system which shows a good pairing with the digestive system. This is because for energy to be synthesized by oxidative phosphorylation, respiratory systems need the supply of Oxygen. The oxygen is needed to accept the final electron in the synthesis of ATPs during oxidative phosphorylation.

This process begins with substrate level phosphorylation-glycolysis - the breaking down of glucose from the digestive system.

Hence, respiratory system needs the supply of oxygen for the metabolisms of glucose obtained from digestive system to generate energy (extract the energy in glucose) for the body.