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The reduction in the rate of photosynthesis in monochromatic light of more than 680 nm is called red drop.

1. Red drop occurs in monochromatic light of more than 680 nm.
2. Red drop is a reduction in the rate of photosynthesis.
3. Red drop is observed in the absence of PSII.
4. Red drop is a phenomenon related to the Emerson effect.

Correct answer: Red drop occurs in monochromatic light of more than 680 nm.

Solution

The term “red drop” specifically describes the sharp decrease in photosynthetic rate when plants are exposed to monochromatic light longer than about 680 nm. That matches option A exactly; the other options are either too general or refer to related but different concepts.

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