Final answer:
Among the given wavelengths, plants containing only chlorophyll b would harness the most photosynthetic energy from 450nm (blue light) since it lies within the optimal range of 400 nm to 700 nm for visible light absorption by chlorophylls.
Step-by-step explanation:
Plants that contain chlorophyll b are able to harness energy for photosynthesis by absorbing specific wavelengths of light. According to the wavelengths provided - 10nm (x-rays), 450nm (blue light), 670nm (red light), and 800nm (infrared light) - the plants would harness the most energy from the 450nm (blue light) wavelength. This is because chlorophyll b, along with chlorophyll a, primarily absorbs light in the range of 400 nm to 700 nm, which is the range of visible light.
While shorter wavelengths like X-rays carry more energy, they're not useful for photosynthesis since they can damage cells and DNA. Similarly, wavelengths longer than 700 nm, such as far-red and infrared light, do not support photosynthesis effectively due to lower energy levels and thus, are less likely to be absorbed by the chlorophyll b pigment for photosynthesis. Therefore, within the given wavelengths, blue light at 450 nm is most effective for photosynthetic activity in plants containing only chlorophyll b.