Which of the following statements is incorrect?

To determine which statement is incorrect, let us analyze each option step-by-step:

1. ATP is synthesized through complex V:
This statement is correct. Complex V of the Electron Transport Chain (ETC) is ATP Synthase. It utilizes the proton gradient established across the inner mitochondrial membrane to synthesize ATP from ADP and inorganic phosphate (Pi).

2. Oxidation-reduction reactions produce proton gradient in respiration:
This statement is correct. As electrons pass through the carriers of the Electron Transport Chain via a series of oxidation-reduction (redox) reactions, protons ($H^{+}$) are pumped from the mitochondrial matrix into the intermembrane space, creating a proton gradient.

3. During aerobic respiration, role of oxygen is limited to the terminal stage:
This statement is correct. Oxygen acts as the final electron acceptor at the very end of the Electron Transport Chain (Complex IV), where it reacts with protons to form water. Although oxygen is vital for the entire process to continue, its chemical participation is restricted to this final/terminal stage.

4. In ETC, one molecule of NADH + H⁺ gives rise to 2 ATP molecules, and one FADH₂ gives rise to 3 ATP molecules:
This statement is incorrect. During biological oxidation in the electron transport chain, the oxidation of one molecule of NADH + H⁺ gives rise to 3 molecules of ATP, whereas the oxidation of one molecule of FADH₂ gives rise to 2 molecules of ATP. The values are reversed in the statement.

Therefore, the incorrect statement is the one concerning the ATP yield of NADH + H⁺ and FADH₂.