Final answer:
The Electron Transport Chain (ETC) is the process using NADH and FADH2 to produce ATP. This occurs after the Krebs Cycle, where these electron carriers are generated, and results in a large amount of ATP through oxidative phosphorylation.
Step-by-step explanation:
The process that uses NADH and FADH2 to produce ATP is the Electron Transport Chain (ETC), identified as option a. In the electron transport chain, NADH and FADH2, which were produced during earlier stages of cellular respiration such as the Krebs Cycle, donate electrons. These electrons are used to pump protons across the inner mitochondrial membrane, creating a proton gradient that facilitates the synthesis of ATP through a process known as chemiosmosis. This represents the final stage of oxidative phosphorylation, where most of the ATP during cellular respiration is generated.
The citric acid cycle (also known as the Krebs Cycle) is where NADH and FADH2 are initially produced. These electron carriers are later utilized in the ETC to create a large amount of ATP. In terms of the quantity of ATP, approximately three ATP molecules are produced for every NADH molecule oxidized, and about two ATP for every FADH2 oxidized within the ETC. Thus, the correct option is a. ETC, where the harvested energy from NADH and FADH2 is transformed into the usable energy currency of the cell, ATP, through oxidative phosphorylation.