Answer:
The electron transport chain is a cycle or procession of various complexes that able to transfer electrons from electron donors and electron accepts via oxidation-reduction reaction simultaneously and with the help of this electron transfer with transfer of protons through the membrane.
In eukaryotes, the electron transport chain is made up of various series of proteins present in the mitochondria's inner membranes whereas in prokaryotes it is taking place in the cell membrane.
Thus, the correct answer is - ETC occurs in the cell membrane whereas in the eukaryotes it takes place in the inner membrane of the mitochondria.