Final answer:
Plants containing only chlorophyll b will harness the most energy for photosynthesis when exposed to 450nm (blue light), as it falls within the optimal range known as photosynthetically active radiation (400-700nm).
Step-by-step explanation:
Plants harness energy for photosynthesis primarily using chlorophyll pigments, which absorb light in specific wavelengths. Among the wavelengths you have listed, chlorophyll b absorbs light most efficiently at around 450nm (blue light) and 675nm (close to the 670nm red light provided in your options). This is because pigments like chlorophyll are adapted to absorb light in the range known as photosynthetically active radiation, which is between 700 nm and 400 nm. So, plants containing only chlorophyll b would harness the most energy for photosynthesis when exposed to 450nm, since it falls within the optimal range for absorption by this pigment.
X-rays (10nm) have too much energy and can damage the molecules through bleaching. Infrared light (800nm) carries too little energy to excite the electrons necessary for photosynthesis. The 670nm red light is close to the absorbance peak of chlorophyll b but less optimal than the blue light at 450nm. As a result, plants exposed to 450nm blue light are predicted to harness the most energy for photosynthesis.