Emerson's enhancement effect and red drop have been instrumental in the discovery of

Emerson's enhancement effect and red drop have been instrumental in the discovery of
A two photosystems operating simultaneously.
B photophosphorylation and cyclic electron transport.
C oxidative phosphorylation.
D photophosphorylation and non-cyclic electron transport.

Explanation

Red drop and enhancement revealed PS I and PS II.

Detailed Solution

The quantum yield dropped sharply when $Chlorella$ was illuminated with red light of wavelength more than 680 nm ('red drop'). When it was irradiated with both 680 nm and above 700 nm light, the rate of photosynthesis increased ('enhancement effect').
This showed there are two reaction centres (photosystems I and II) operating simultaneously.

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