Nitrogen fixation is carried out by the nitrogenase enzyme system, which is really a two protein machine, and this question is only asking you to place three names, ferredoxin, reductase, and nitrogenase, in the right order along the electron path to $N_2$.
- Ferredoxin to reductase to nitrogenase: ferredoxin is the small electron carrier kept reduced by the cell's metabolism; it hands its electrons to the reductase protein (also called the Fe protein or dinitrogenase reductase), which then, using energy from ATP hydrolysis, transfers electrons one at a time to nitrogenase (the MoFe protein), where $N_2$ is bound and reduced. This matches the accepted biochemical pathway exactly.
- NADH to reductase to nitrogenase: NADH is not the physiological electron donor feeding the reductase component in the standard nitrogenase pathway; that role belongs to ferredoxin (or flavodoxin in some bacteria), so this order is not correct.
- Ferredoxin to nitrogenase to reductase: this places nitrogenase before reductase, but nitrogenase (the MoFe protein) cannot accept electrons straight from ferredoxin; it only receives them after the reductase component has processed them, so the order is reversed.
- Reductase to NADH to nitrogenase: this sequence has no basis at all in the real pathway; reductase does not hand electrons to NADH, and NADH is not the donor to nitrogenase.
Only the first sequence, ferredoxin to reductase to nitrogenase, reflects how electrons genuinely travel in the nitrogenase system before $N_2$ is reduced to $NH_3$, so option (A) is correct.
Let's summarize:
- Ferredoxin is the primary electron donor to the nitrogenase system in most nitrogen fixing organisms.
- Electrons must pass through the reductase (Fe protein) before reaching nitrogenase (MoFe protein), never the reverse.
So the correct order of electron flow is ferredoxin, then reductase, then nitrogenase, option (A).