The correct option is B Complex II
The electron transport chain is a collection of membrane-embedded proteins and organic molecules, most of them organized into four large complexes labeled I to IV.
In eukaryotes, many copies of these molecules are found in the inner mitochondrial membrane.
Complexes I, III, and IV of the electron transport chain are proton pumps.
As electrons move energetically downhill, the complexes capture the released energy and use it to pump H ions from the matrix to the intermembrane space.
This pumping forms an electrochemical gradient across the inner mitochondrial membrane, which is utilized by ATP synthase to generate ATP.