Answer:
C. Proton gradient
Step-by-step explanation:
During electron transport chain, the electrons are comes from transport chain come from NADH and FADH₂. As electrons are travels down the chain, proton gradient develops across the inner mitochondrial membrane due to accumulation of high proton (H⁺) in the intermembrane space and a lower proton(H⁺) in the matrix. This proton gradient is used to syntheis ATP.