Not all light wavelengths penetrate equally in water. consequently, plants that live relatively deep in the sea have leaves that look reddish when brought to the surface. based on this information, which part of the light spectrum (purple, blue, green, yellow, orange, red) penetrate the best?

The part of the light spectrum that penetrate best is the BLUE LIGHT. 
As one moves from the surface water to the deeper water, light changes in quantity and quality. The quantity of light decreases with depth, so also is the wavelength of light. As the wavelength of light decreases form red to blue light, so does the ability of light to penetrate water. Thus, blue light penetrate best followed by green, yellow, orange and red light.

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