Answer
C.
Work Step by Step
Electron transport builds up the proton gradient which ATP synthase uses to make ATP (C.). A. is wrong because it refers to an earlier intermediate in the overall breakdown of glucose, and B. also refers to something other than the actual powering of ATP synthase. D. can be excluded because for the whole process to work, including electron transport and ATP synthesis, oxygen is needed, which contradicts what D. says.
