The formation of ATP by oxidative phosphorylation takes place in...

Oxidative phosphorylation is the process in which ATP is formed as a result of the transfer of electrons from NADH or FADH_2$to O_2$ by a series of electron carriers. This process takes place in the Electron Transport System (ETS) located in the inner mitochondrial membrane.

In glycolysis and the Krebs cycle, ATP is formed through substrate-level phosphorylation.

Fermentation is an anaerobic process that does not involve oxidative phosphorylation.

Answer: ETS