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Cyclic photophosphorylation also occurs when

(a) only light beyond 680 nm is available Correct
(b) the CO₂ concentration is very high Your answer
(c) only blue light below 500 nm is available Your answer
(d) the plant is kept in complete darkness Your answer

Your answer: Correct answer: (a)

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Explanation

Why (a) is correct

Cyclic photophosphorylation also occurs when only light of wavelength beyond 680 nm is available, because then only PS I is excited.

Why the other options are wrong

(b) · your answer High CO₂ does not switch the plant to cyclic flow.
(c) · your answer Blue light excites both photosystems.
(d) · your answer No light-driven electron flow happens in complete darkness.

NCERT point: Far-red light drives PS I only.

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