Because they contain a green pigment called chlorophyll.
according to physics not biology
This answer is correct, but I need the answer according to physics(light)
see there
Oh, okay. Sorry.
plants are green because their cells contain chloroplasts which have the pigment chlorophyll which absorbs deep-blue and red light, so that the rest of the sunlight spectrum is being reflected, causing the plant to look green. I am talking about the methodological reason why plants do not take advantage of the green light, since sunlight emits the highest light intensity in the green spectrum. 
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Thanks a lot! Thank you...