Answer:
Option A , Electron transport chain
Explanation:
In Electron transport chain(ETC), no carbon based reactant or product are involved.
Usually in ETC (taking place in the inner membrane of the mitochondria), NADH and FADH2 releases high-energy electrons. Some energy of these electrons is capture while passing through the chain. This stored energy is then used for pumping hydrogen ions into the intermembrane space from the matrix.
Since in this entire process no carbon element is involved and hence, option A is correct
