The correct option is A drop in quantum yield
In the process of photosynthesis, two photosystems (PS1 and PS2) are involved in the entrapment of sunlight. PS1 absorb light of wavelength 700nm while PS2 absorbs light in the wavelength of 680nm.
When the chloroplast is exposed to the light of wavelength 680nm and 700nm, maximum photosynthesis is observed. This effect is known as the Emerson effect.
When the available wavelength is beyond 680nm, the efficiency of photosynthesis decreases because PS1 stops performing. This effect is called the red drop effect. In other words, when the wavelength is greater than 680nm, there is a sharp decrease in quantum yield.