The coolest stars have wavelengths between 400nm and 500 nm. Thus the correct option is D.
What is wavelength?
Wavelength is defined as the distance between identical points (adjacent crests) in the adjacent cycles of a waveform signal propagated in space or along a wire.
Based on the graph, wavelengths at approximately 500mn has less chance or can not emit light at the red part. Cool stars emit most of their visible light energy at red wavelengths (with more radiation coming off in the infrared).
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