The correct answer is proteins.
The oxidative phosphorylation in the eukaryotes takes place within the mitochondria with the help of electron transport chain. The main objective of the electron transport chain is to accumulate enough amount of protons in the intermembrane space so that there will be a concentration gradient which would help in the production of ATP.
The electron transport chain comprises of various molecules, but majorly the proteins, which are situated in the inner membrane of the mitochondria.