Which one of the following mode of action of herbicides is different from the other?

Correct option is B

Inhibit protein synthesis is different from the other modes listed, as it targets the ribosomes and translation process inside the cell, affecting the biosynthesis of proteins.
The other three – Hill reaction, electron transfer, and CO₂ fixation – are directly related to photosynthesis and occur within the chloroplasts.
Herbicides that inhibit protein synthesis act at the cellular level, not just within the photosynthetic machinery.
Explanation:
(a) Hill reaction – Involves photolysis of water during photosynthesis; targeted by some herbicides like Atrazine.
(b) Inhibit protein synthesis – Correct answer. Targets non-photosynthetic cellular processes, like translation, making it mechanistically different.
(c) Electron transfer – A step in the light-dependent reactions of photosynthesis, disrupted by photosystem II inhibitors.
(d) CO₂ fixation – Refers to the Calvin cycle or C4 pathway in photosynthesis, also targeted by some herbicides.