Oxidative phosphorylation:
Oxidative phosphorylation is the final stage of cellular respiration, which occurs in the mitochondria of eukaryotic cells. It is responsible for producing the majority of ATP in aerobic organisms.

The electron transport chain:
The end-product of oxidative phosphorylation is ATP and H2O. This process involves the electron transport chain (ETC), which is a series of protein complexes located in the inner mitochondrial membrane. The ETC receives electrons from NADH and FADH2, which are produced during the previous stages of cellular respiration (glycolysis and the citric acid cycle).

Complexes of the electron transport chain:
1. Complex I (NADH dehydrogenase): This complex accepts electrons from NADH and transfers them to coenzyme Q (ubiquinone), while also pumping protons across the inner mitochondrial membrane.

2. Complex II (succinate dehydrogenase): This complex accepts electrons from FADH2, which is produced during the citric acid cycle. Complex II also transfers the electrons to coenzyme Q.

3. Complex III (cytochrome bc1 complex): This complex accepts electrons from coenzyme Q and transfers them to cytochrome c, while pumping protons across the inner mitochondrial membrane.

4. Complex IV (cytochrome c oxidase): This complex accepts electrons from cytochrome c and transfers them to molecular oxygen (O2), which is the final electron acceptor. Complex IV also pumps protons across the inner mitochondrial membrane.

ATP synthesis:
The movement of electrons through the ETC generates a proton gradient across the inner mitochondrial membrane. This gradient drives the synthesis of ATP by ATP synthase, a protein complex embedded in the inner mitochondrial membrane.

ATP and water production:
ATP synthase uses the energy from the proton gradient to convert ADP (adenosine diphosphate) and inorganic phosphate (Pi) into ATP (adenosine triphosphate). This process is known as oxidative phosphorylation.

At the end of the electron transport chain, molecular oxygen (O2) accepts electrons and combines with protons to form water (H2O). This is the final step of oxidative phosphorylation and cellular respiration, where water is produced as a byproduct.

Therefore, the correct answer is option 'A': ATP and H2O.