Why can electrons cannot flow in the reverse direction in the electron transport chain.

Reverse electron flow (also known as reverse electron transport) is a mechanism in microbial metabolism. ... This energy is supplied by consuming proton motive force to drive electrons in a reverse direction through an electron transport chain and is thus the reverse process as forward electron transport.

the relative standard reduction potential of the half-cell in which reduction occurs; more positive than the other half-cell. reduction is a chemical process in which electrons are added to an atom or an ion; it always occurs accompanied by oxidation of the reducing agent.

i think the answer is b.