Final answer:
Green light is used least for photosynthesis because it is reflected rather than absorbed by chlorophyll, making it less effective for the process compared to red and blue light.
Step-by-step explanation:
The color of light that is used least for photosynthesis is green. This is because plants are green due to the reflection of green wavelength by chlorophyll, the main pigment involved in photosynthesis. The green light is not absorbed effectively and is thus used the least. Chlorophyll prefers to absorb light in the red and blue wavelengths. Plant physiologists define the range of light that is effectively absorbed for photosynthesis as photosynthetically active radiation, which includes wavelengths from 700 nm (red) to 400 nm (blue).
To illustrate, plants containing only chlorophyll b absorb light best at wavelengths different from green. Similarly, when considering the growth of plants under different colors of light, the plant grown under green light (Plant B) would show the least growth as it would perform photosynthesis less efficiently compared to the ones grown under blue and orange light.