Final answer:
The 3rd part of the aerobic respiration cycle is the (Option c) Electron Transport Chain (ETC), where energy from NADH and FADH2 is used to produce ATP by chemiosmosis.
Step-by-step explanation:
The 3rd part of the aerobic respiration cycle is c. Electron Transport Chain (ETC). During the ETC, energy carried by molecules like NADH and FADH2, generated from previous steps such as glycolysis and the Krebs cycle, is used to produce ATP. The electron transport chain functions by passing these electrons through a series of reactions, similar to what occurs during photosynthesis, driving the production of ATP through a process known as chemiosmosis.