Final answer:
Cytochrome c does not act as a proton pump in the electron-transport chain; it serves as a mobile electron carrier between Complex III and Complex IV.
Step-by-step explanation:
The component of the electron-transport chain that does not act as a proton pump is (b) cytochrome c. This component is a mobile electron carrier that transfers electrons between Complex III (cytochrome c reductase) and Complex IV (cytochrome c oxidase) in the mitochondrial inner membrane. Both CoQ (ubiquinone) and cytochrome c do not directly pump protons across the membrane; instead, they facilitate the transfer of electrons within the electron transport chain. Complexes I (NADH dehydrogenase), III (cytochrome c reductase), and IV (cytochrome c oxidase) are the ones involved in pumping protons to generate the proton gradient necessary for ATP synthesis.