The correct option is D Increase in photosynthesis when lights of two different wavelengths are provided together.
Emerson effect was observed by Robert Emerson in 1957. Emerson effect shows that photosynthesis proceeding very slowly when the light of 700 nm wavelength is used can be increased by illuminating with light of shorter wavelength i.e. 650 nm. So, Emerson effect is related to increase in photosynthesis when lights of two different wavelengths are provided together. Emerson effect is also known as enhancement effect. It is evident of the presence of two photosystems i.e. Photosystems I and II.